feat: sync notebooks between devices via a synced folder (Issue #49, stage 1)

- Storage location setting: the notebooks can be moved into a folder kept in
  sync by Nextcloud, Google Drive, Syncthing etc. (subfolder `ink-notes`);
  existing notebooks are copied there if it is still empty, the originals stay.
- Changes from other devices are taken over while everything local is saved:
  changed pages are reloaded, new/removed/reordered pages and titles come in
  through the manifest. A page file still being synced is retried later
  instead of being set aside as damaged.
- Conflict copies made by sync clients become pages of their own.
- `.opened_by.json` marks the device that has a notebook open; opening a
  notebook open elsewhere shows a warning.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.de.md

@@ -68,6 +68,7 @@ Die Oberfläche gibt es auf Deutsch und Englisch; ink-notes folgt der Systemspra
 - Jede Datei wird erst vollständig in eine temporäre Datei geschrieben und dann ersetzt – ein Absturz beim Speichern hinterlässt keine halbe Datei.
 - **Beim Schließen** (Fenster oder `Strg + Q`) und beim Wechsel des Notizbuchs wird alles Ausstehende geschrieben, bevor es weitergeht. Schlägt das fehl, bleibt die App offen und bietet Wiederholen oder eine Notfall-Sicherung an.
 - `Strg + S` speichert sofort. Der Punkt am ☰-Knopf zeigt den Stand: grün = gespeichert, orange = Änderungen warten, gelb = speichert gerade, rot = Fehler.
+- **Speicherort** (☰-Menü, „Speicherort“): Liegen die Notizbücher in einem Ordner, den **Nextcloud, Google Drive, Syncthing oder Dropbox** abgleichen, sind sie auf allen Geräten verfügbar. Die Notizbücher liegen im Unterordner `ink-notes`; ist er noch leer, werden die vorhandenen Notizbücher dorthin kopiert (die alten bleiben erhalten). Änderungen von einem anderen Gerät erscheinen nach wenigen Sekunden ohne neu zu öffnen – aber nur, solange lokal alles gespeichert ist, damit nichts überschrieben wird. Legt der Sync-Client eine Konfliktkopie einer Seite an, wird sie als eigene Seite („Konfliktkopie: …“) übernommen statt verloren zu gehen. Ist ein Notizbuch gerade auf einem anderen Gerät geöffnet, warnt die App beim Öffnen.
 
 ### Export
 - **PDF** (`Strg + E`) nach `~/Documents/<Titel>.pdf`: Striche und Formen als Vektoren, Textfelder als echter, durchsuchbarer Text, importierte PDF-Seiten als Vektor-Unterlage, Seitentitel als Lesezeichen.
@@ -178,7 +179,8 @@ src/
 ├── saber.rs        # Import von Saber-Notizbüchern
 ├── document.rs     # Datenmodell: Notizbuch, Seiten, Striche, Seitengrößen
 ├── history.rs      # Rückgängig/Wiederholen
-├── storage.rs      # Laden/Speichern (atomar), Notizbücher, Papierkorb
+├── storage.rs      # Laden/Speichern (atomar), Notizbücher, Papierkorb, Speicherort
+├── disk_watch.rs   # Änderungen anderer Geräte übernehmen (Sync-Clients)
 ├── library.rs      # Ordner der Notizbücher (library.json)
 ├── autosave.rs     # Speichern im Hintergrund
 ├── export.rs       # PDF-Export (printpdf)

README.md

@@ -68,6 +68,7 @@ The interface is available in English and German; ink-notes follows the system l
 - Every file is first written completely to a temporary file and then replaced – a crash while saving never leaves a half-written file.
 - **When closing** (window or `Ctrl + Q`) and when switching notebooks, everything pending is written before continuing. If that fails, the app stays open and offers to retry or to make an emergency backup.
 - `Ctrl + S` saves immediately. The dot on the ☰ button shows the state: green = saved, orange = changes pending, yellow = saving, red = error.
+- **Storage location** (☰ menu, "Storage location"): put the notebooks into a folder that **Nextcloud, Google Drive, Syncthing or Dropbox** keep in sync, and they are available on all your devices. The notebooks are kept in the subfolder `ink-notes`; if it is still empty, the existing notebooks are copied there (the old ones stay). Changes arriving from another device show up within a few seconds without reopening – but only while everything local is saved, so nothing gets overwritten. If the sync client makes a conflict copy of a page, it is added as a page of its own („Conflict copy: …“) instead of getting lost. Opening a notebook that is open on another device shows a warning.
 
 ### Export
 - **PDF** (`Ctrl + E`) to `~/Documents/<title>.pdf`: strokes and shapes as vectors, text boxes as real, searchable text, imported PDF pages as vector background, page titles as bookmarks.
@@ -180,7 +181,8 @@ src/
 ├── saber.rs        # Import of Saber notebooks
 ├── document.rs     # Data model: notebook, pages, strokes, page sizes
 ├── history.rs      # Undo/redo
-├── storage.rs      # Loading/saving (atomic), notebooks, trash
+├── storage.rs      # Loading/saving (atomic), notebooks, trash, storage location
+├── disk_watch.rs   # Taking over changes from other devices (sync clients)
 ├── library.rs      # Notebook folders (library.json)
 ├── autosave.rs     # Saving in the background
 ├── export.rs       # PDF export (printpdf)

docs/FILE_FORMAT.de.md

@@ -21,6 +21,7 @@ Dieses Dokument beschreibt, wo und wie ink-notes Notizbücher speichert, und die
 │   │   ├── assets/                    eingefügte Bilder und importierte PDFs
 │   │   │   ├── <sha256>.png
 │   │   │   └── <sha256>.pdf
+│   │   ├── .opened_by.json            Gerät, auf dem das Notizbuch geöffnet ist (siehe unten)
 │   │   └── .lock                      Sperrdatei, solange das Notizbuch geöffnet ist
 │   └── .trash/                        gelöschte Notizbücher (<id>_<zeitstempel>)
 └── recovery/                          Notfall-Sicherungen ungespeicherter Seiten
@@ -28,6 +29,8 @@ Dieses Dokument beschreibt, wo und wie ink-notes Notizbücher speichert, und die
 ~/.config/ink-notes/ui.json            ($XDG_CONFIG_HOME) Einstellungen der Oberfläche
 ```
 
+Der Ordner `notebooks/` lässt sich in den Einstellungen verlegen (`storage_dir` in `ui.json`), z. B. nach `~/Nextcloud/ink-notes`; `recovery/` bleibt, wo es ist.
+
 - **Notizbuch-IDs** werden aus dem Titel und dem Zeitpunkt gebildet (z. B. `mathematik_1775460000`); das erste Notizbuch heißt `default`.
 - **Assets** sind nach dem SHA-256-Hash ihres Inhalts benannt; dasselbe Bild wird nur einmal gespeichert. Bilder werden als PNG oder (unverändert) JPEG abgelegt, PDFs unverändert.
 - Alle Dateien werden mit Rechten `0600`, Ordner mit `0700` angelegt.
@@ -167,6 +170,12 @@ Ein Absturz während des Speicherns hinterlässt deshalb entweder die alte oder
 ### Gelöschte Seiten
 Eine gelöschte Seite verschwindet aus `manifest.json`; ihre Datei `page_<id>.json` bleibt im Ordner liegen. Seitenkennungen werden nie wiederverwendet (`next_page_id`), die Datei kann also mit keiner neuen Seite kollidieren. Wird die Seite in derselben Sitzung wiederhergestellt, schreibt die App sie neu.
 
+### Abgleich zwischen Geräten
+Liegen die Notizbücher in einem Ordner, den Nextcloud, Google Drive, Syncthing o. Ä. abgleichen:
+- `.opened_by.json` (`{"device": "<Rechnername>", "since": <Unix-Zeit>}`) wird beim Öffnen eines Notizbuchs geschrieben und beim Schließen entfernt. Eine Markierung eines anderen Geräts führt zu einer Warnung; die Sperrdatei `.lock` schützt nur vor einer zweiten Instanz auf demselben Rechner.
+- Solange alle lokalen Änderungen gespeichert sind, prüft die App den Ordner alle 3 Sekunden (und wenn das Fenster den Fokus bekommt). Geänderte Seitendateien werden neu geladen, ein geändertes `manifest.json` bringt neue, entfernte oder umsortierte Seiten und Titel mit.
+- Konfliktkopien der Sync-Clients (`page_<id> (….json`, `page_<id>_….json`, `page_<id>.sync-conflict-….json`) werden als neue Seiten hinter dem Original eingefügt und in `<name>.imported` umbenannt.
+
 ### Neuere Versionen
 Dateien mit einer höheren `format_version` als der unterstützten werden nicht geöffnet – so kann eine ältere App-Version keine Inhalte verlieren, die sie nicht kennt.
 

docs/FILE_FORMAT.md

@@ -21,6 +21,7 @@ This document describes where and how ink-notes stores notebooks, and the versio
 │   │   ├── assets/                    inserted images and imported PDFs
 │   │   │   ├── <sha256>.png
 │   │   │   └── <sha256>.pdf
+│   │   ├── .opened_by.json            device that has the notebook open (see below)
 │   │   └── .lock                      lock file while the notebook is open
 │   └── .trash/                        deleted notebooks (<id>_<timestamp>)
 └── recovery/                          emergency backups of unsaved pages
@@ -28,6 +29,8 @@ This document describes where and how ink-notes stores notebooks, and the versio
 ~/.config/ink-notes/ui.json            ($XDG_CONFIG_HOME) interface settings
 ```
 
+The folder `notebooks/` can be moved elsewhere in the settings (`storage_dir` in `ui.json`), e.g. to `~/Nextcloud/ink-notes`; `recovery/` stays where it is.
+
 - **Notebook IDs** are built from the title and the time (e.g. `mathematik_1775460000`); the first notebook is called `default`.
 - **Assets** are named after the SHA-256 hash of their content; the same image is stored only once. Images are stored as PNG or (unchanged) JPEG, PDFs unchanged.
 - All files are created with permissions `0600`, folders with `0700`.
@@ -167,6 +170,12 @@ A crash while saving therefore leaves either the old or the new file, never a ha
 ### Deleted pages
 A deleted page disappears from `manifest.json`; its file `page_<id>.json` stays in the folder. Page IDs are never reused (`next_page_id`), so the file cannot collide with a new page. If the page is restored in the same session, the app writes it again.
 
+### Syncing between devices
+If the notebooks are kept in a folder synced by Nextcloud, Google Drive, Syncthing or similar:
+- `.opened_by.json` (`{"device": "<host name>", "since": <unix time>}`) is written when a notebook is opened and removed when it is closed. A marker from another device leads to a warning; the lock file `.lock` only protects against a second instance on the same computer.
+- While all local changes are saved, the app checks the folder every 3 seconds (and when the window gets the focus). Changed page files are reloaded, a changed `manifest.json` brings in new, removed or reordered pages and titles.
+- Conflict copies of sync clients (`page_<id> (….json`, `page_<id>_….json`, `page_<id>.sync-conflict-….json`) are added as new pages after the original and renamed to `<name>.imported`.
+
 ### Newer versions
 Files with a higher `format_version` than supported are not opened – this way an older app version cannot lose content it does not know.
 

src/app.rs

@@ -22,6 +22,7 @@ use crate::assets::{AssetStore, ImportedImage};
 use crate::autosave::{AutosaveController, AutosaveStatus};
 use crate::background::PatternColor;
 use crate::canvas::CanvasState;
+use crate::disk_watch::DiskWatch;
 use crate::document::{Notebook, PageBackground};
 use crate::export::{self, export_notebook_to_pdf, ExportStrategy};
 use crate::geometry::{self, RecognizedShape};
@@ -214,6 +215,16 @@ fn save_ui_settings(settings: &UiSettings, path: Option<&std::path::Path>, ui_st
     }
 }
 
+/// Warns when the notebook is still marked as open on another device (Issue #49).
+fn warn_if_open_elsewhere(lock: Option<&storage::NotebookLock>, ui_state: &mut UiState) {
+    if let Some(marker) = lock.and_then(|l| l.open_elsewhere.as_ref()) {
+        ui_state.set_export_message(t!(
+            "⚠ Auch auf '{device}' geöffnet – Änderungen dort können deine überschreiben",
+            device = marker.device
+        ));
+    }
+}
+
 fn update_window_title(
     window: &Window,
     notebook: &Notebook,
@@ -1308,7 +1319,10 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
     let mut ime_composing = false;
 
     // 1. Notizbuch aus Storage laden oder initialisieren (Session-Wiederherstellung)
-    let storage_base_dir = storage::default_storage_dir();
+    let mut storage_base_dir = ui_settings
+        .storage_dir
+        .clone()
+        .unwrap_or_else(storage::default_storage_dir);
     let _ = storage::cleanup_trash(
         &storage_base_dir,
         std::time::Duration::from_secs(30 * 24 * 3600),
@@ -1408,6 +1422,8 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
     ui_state.document_title = manifest.title.clone();
     ui_state.available_notebooks = available_notebooks;
     ui_state.library = Library::load(&storage_base_dir);
+    ui_state.storage_dir = storage_base_dir.clone();
+    ui_state.storage_dir_custom = ui_settings.storage_dir.is_some();
     if let Some(warning) = startup_warning {
         refresh_notebooks(&storage_base_dir, &mut ui_state);
         ui_state.set_export_message(warning);
@@ -1426,6 +1442,7 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
     let mut _notebook_lock = match storage::NotebookLock::acquire(&storage_dir) {
         Ok(lock) => {
             autosave.set_read_only(false);
+            warn_if_open_elsewhere(Some(&lock), &mut ui_state);
             Some(lock)
         }
         Err(storage::StorageError::AlreadyLocked(msg)) => {
@@ -1441,6 +1458,9 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
         }
     };
 
+    // Notices changes made on other devices through a sync client (Issue #49)
+    let mut disk_watch = DiskWatch::new(storage_dir.clone(), Instant::now());
+
     // Initiale Seite und Werkzeugeinstellungen mit Canvas synchronisieren
     canvas.set_paper_theme(ui_state.paper_theme, ui_state.adapt_ink_colors);
     canvas.set_pattern_color(ui_state.pattern_color);
@@ -1616,6 +1636,50 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                     update_window_title(&window, &notebook, &manifest, ui_state.active_tool, new_status);
                 }
 
+                // Take over changes from other devices (Issue #49), but only while everything
+                // local is saved and nothing is being drawn or typed
+                if disk_watch.dir != storage_dir {
+                    disk_watch = DiskWatch::new(storage_dir.clone(), Instant::now());
+                }
+                let now = Instant::now();
+                if disk_watch.is_due(now) {
+                    let idle = new_status == AutosaveStatus::Saved
+                        && !is_drawing
+                        && !is_stylus_drawing
+                        && ui_state.text_edit.is_none();
+                    if !idle {
+                        disk_watch.postpone(now);
+                    } else {
+                        let outcome = disk_watch.check(&mut notebook, &mut manifest, now);
+                        if !outcome.is_empty() {
+                            if outcome.reloaded_pages > 0 || outcome.structure_changed || !outcome.conflict_pages.is_empty() {
+                                selection_state.clear();
+                                canvas.sync_page(notebook.current_page_mut());
+                                ui_state.document_title = manifest.title.clone();
+                                ui_state.set_export_message(t!("Änderungen von einem anderen Gerät übernommen").to_string());
+                            }
+                            if !outcome.conflict_pages.is_empty() {
+                                for page in notebook.pages.iter().filter(|p| outcome.conflict_pages.contains(&p.id)) {
+                                    autosave.mark_page_dirty(page, &manifest);
+                                }
+                                autosave.mark_manifest_dirty(&manifest);
+                                ui_state.set_export_message(t!(
+                                    "Konfliktkopien als eigene Seiten übernommen: {count}",
+                                    count = outcome.conflict_pages.len()
+                                ));
+                            }
+                            if let Some(marker) = &outcome.opened_elsewhere {
+                                ui_state.set_export_message(t!(
+                                    "⚠ Gerade auf '{device}' geöffnet – Änderungen dort können deine überschreiben",
+                                    device = marker.device
+                                ));
+                            }
+                            update_window_title(&window, &notebook, &manifest, ui_state.active_tool, autosave.status());
+                            window.request_redraw();
+                        }
+                    }
+                }
+
                 while let Ok(result) = export_rx.try_recv() {
                     match result {
                         Ok(path) => ui_state.set_export_message(t!("Gespeichert: {path}", path = path.display())),
@@ -2149,7 +2213,7 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                     || fling.is_active()
                 {
                     target.set_control_flow(ControlFlow::wait_duration(Duration::from_millis(8)));
-                } else if let Some(at) = [egui_repaint_at, long_press.deadline()]
+                } else if let Some(at) = [egui_repaint_at, long_press.deadline(), Some(disk_watch.deadline())]
                     .into_iter()
                     .flatten()
                     .min()
@@ -2234,6 +2298,12 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                         window.request_redraw();
                     }
 
+                    WindowEvent::Focused(true) => {
+                        // Coming back from another app: look for changes right away
+                        disk_watch.check_soon(Instant::now());
+                        window.request_redraw();
+                    }
+
                     WindowEvent::Focused(false) => {
                         // Bei Fokusverlust Strich abbrechen und 1€-Filter resetten
                         if is_drawing {
@@ -3828,6 +3898,7 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                                                 if _notebook_lock.is_some() {
                                                     ui_state.set_export_message(t!("Dokument '{title}' geöffnet", title = manifest.title));
                                                 }
+                                                warn_if_open_elsewhere(_notebook_lock.as_ref(), &mut ui_state);
                                                 window.request_redraw();
                                             }
                                             Err(err) => {
@@ -3991,6 +4062,7 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                                                             canvas.sync_page(notebook.current_page_mut());
                                                             update_window_title(&window, &notebook, &manifest, ui_state.active_tool, autosave.status());
                                                             ui_state.set_export_message(t!("Gewechselt zu '{title}'", title = manifest.title));
+                                                            warn_if_open_elsewhere(_notebook_lock.as_ref(), &mut ui_state);
                                                         }
                                                         Err(err) => {
                                                             eprintln!("[Dokument-Fehler] Konnte Dokument '{}' nicht laden: {err}", first.id);
@@ -4127,6 +4199,77 @@ pub fn run() -> Result<(), Box<dyn std::error::Error>> {
                                     save_ui_settings(&ui_settings, ui_settings_path.as_deref(), &mut ui_state);
                                     window.request_redraw();
                                 }
+                                UiAction::SetStorageDir(chosen) => {
+                                    let new_base = match &chosen {
+                                        Some(dir) => storage::storage_dir_in(dir),
+                                        None => storage::default_storage_dir(),
+                                    };
+                                    if new_base == storage_base_dir {
+                                        ui_state.set_export_message(t!("Die Notizbücher liegen bereits dort").to_string());
+                                    } else {
+                                        let _ = autosave.flush();
+                                        match storage::copy_storage(&storage_base_dir, &new_base) {
+                                            Ok(copied) => {
+                                                // Release lock and open marker in the old location
+                                                _notebook_lock = None;
+                                                storage_base_dir = new_base;
+                                                ui_settings.storage_dir = chosen.map(|_| storage_base_dir.clone());
+                                                save_ui_settings(&ui_settings, ui_settings_path.as_deref(), &mut ui_state);
+                                                ui_state.storage_dir = storage_base_dir.clone();
+                                                ui_state.storage_dir_custom = ui_settings.storage_dir.is_some();
+                                                refresh_notebooks(&storage_base_dir, &mut ui_state);
+                                                // Same notebook if it exists there, otherwise the first one
+                                                let open_id = ui_state
+                                                    .available_notebooks
+                                                    .iter()
+                                                    .filter(|s| s.status == storage::NotebookStatus::Normal)
+                                                    .find(|s| s.id == manifest.id)
+                                                    .or_else(|| ui_state.available_notebooks.iter().find(|s| s.status == storage::NotebookStatus::Normal))
+                                                    .map(|s| s.id.clone());
+                                                let loaded = match open_id {
+                                                    Some(id) => {
+                                                        let dir = storage_base_dir.join(&id);
+                                                        storage::load_notebook(&dir).map(|(nb, m)| (nb, m, dir))
+                                                    }
+                                                    None => storage::create_new_notebook(&storage_base_dir, t!("Standard-Notizbuch")),
+                                                };
+                                                match loaded {
+                                                    Ok((new_nb, new_manifest, new_dir)) => {
+                                                        _notebook_lock = storage::NotebookLock::acquire(&new_dir).ok();
+                                                        autosave.set_read_only(_notebook_lock.is_none());
+                                                        autosave.switch_notebook_dir(new_dir.clone());
+                                                        notebook = new_nb;
+                                                        manifest = new_manifest;
+                                                        storage_dir = new_dir;
+                                                        selection_state.clear();
+                                                        ui_state.current_notebook_id = manifest.id.clone();
+                                                        ui_state.document_title = manifest.title.clone();
+                                                        refresh_notebooks(&storage_base_dir, &mut ui_state);
+                                                        canvas.sync_page(notebook.current_page_mut());
+                                                        update_window_title(&window, &notebook, &manifest, ui_state.active_tool, autosave.status());
+                                                        ui_state.set_export_message(if copied > 0 {
+                                                            t!("Speicherort geändert, {count} Notizbücher kopiert", count = copied)
+                                                        } else {
+                                                            t!("Speicherort geändert: {path}", path = storage_base_dir.display())
+                                                        });
+                                                        warn_if_open_elsewhere(_notebook_lock.as_ref(), &mut ui_state);
+                                                    }
+                                                    Err(err) => {
+                                                        // The old notebook stays open but is saved nowhere
+                                                        autosave.set_read_only(true);
+                                                        eprintln!("[Storage] Notizbuch am neuen Speicherort nicht geöffnet: {err}");
+                                                        ui_state.set_export_message(t!("Fehler beim Öffnen: {err}", err = err));
+                                                    }
+                                                }
+                                            }
+                                            Err(err) => {
+                                                eprintln!("[Storage] Speicherort '{}' nicht nutzbar: {err}", new_base.display());
+                                                ui_state.set_export_message(t!("Speicherort nicht nutzbar: {err}", err = err));
+                                            }
+                                        }
+                                    }
+                                    window.request_redraw();
+                                }
                                 UiAction::SetLanguage(language) => {
                                     ui_settings.language = language;
                                     i18n::set_language(language.unwrap_or_else(Language::system));

src/disk_watch.rs

@@ -0,0 +1,555 @@
+//! Notices changes that another device made to the open notebook through a sync client
+//! such as Nextcloud or Google Drive (Issue #49).
+//!
+//! Every few seconds the notebook folder is scanned (only file times and sizes). Pages whose
+//! file changed are compared with the page in memory and reloaded if they differ; a changed
+//! page order, new or removed pages and titles from the manifest are taken over; conflict
+//! copies made by sync clients (`page_3 (conflicted copy …).json`, `page_3 (1).json`,
+//! `page_3.sync-conflict-….json`) become pages of their own, so nothing is lost. The check
+//! only runs while all local changes are saved, so it never overwrites anything unsaved.
+
+use std::collections::{HashMap, HashSet};
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::time::{Duration, Instant, SystemTime};
+
+use crate::document::{Notebook, Page, PageBackground};
+use crate::storage::{self, NotebookManifest, OpenMarker, PageData};
+
+/// Time between two checks.
+pub const INTERVAL: Duration = Duration::from_secs(3);
+
+type Fingerprint = (Option<SystemTime>, u64);
+
+/// What a check found and took over.
+#[derive(Debug, Default, Clone, PartialEq)]
+pub struct Outcome {
+    /// Pages reloaded because their file changed
+    pub reloaded_pages: usize,
+    /// Pages added, removed, reordered or retitled through the manifest
+    pub structure_changed: bool,
+    /// Ids of the conflict copies added as pages (they need saving)
+    pub conflict_pages: Vec<u64>,
+    /// Another device has just opened the notebook
+    pub opened_elsewhere: Option<OpenMarker>,
+}
+
+impl Outcome {
+    pub fn is_empty(&self) -> bool {
+        *self == Outcome::default()
+    }
+}
+
+pub struct DiskWatch {
+    pub dir: PathBuf,
+    known: HashMap<String, Fingerprint>,
+    /// Files that are neither pages nor conflict copies we could read
+    ignored: HashSet<String>,
+    next_check: Instant,
+}
+
+/// Fingerprints of the JSON files in the notebook folder.
+fn scan(dir: &Path) -> HashMap<String, Fingerprint> {
+    let Ok(entries) = fs::read_dir(dir) else {
+        return HashMap::new();
+    };
+    entries
+        .flatten()
+        .filter_map(|entry| {
+            let name = entry.file_name().to_string_lossy().into_owned();
+            if !name.ends_with(".json") {
+                return None;
+            }
+            let meta = entry.metadata().ok()?;
+            meta.is_file()
+                .then(|| (name, (meta.modified().ok(), meta.len())))
+        })
+        .collect()
+}
+
+/// Page id of a regular page file (`page_<id>.json`).
+fn page_file_id(name: &str) -> Option<u64> {
+    name.strip_prefix("page_")?
+        .strip_suffix(".json")?
+        .parse()
+        .ok()
+}
+
+/// Page id a conflict copy belongs to: `page_<id>` followed by something else.
+pub fn conflict_copy_of(name: &str) -> Option<u64> {
+    if page_file_id(name).is_some() {
+        return None;
+    }
+    let rest = name.strip_prefix("page_")?.strip_suffix(".json")?;
+    let digits: String = rest.chars().take_while(char::is_ascii_digit).collect();
+    let tail = &rest[digits.len()..];
+    let marked =
+        tail.starts_with(" (") || tail.starts_with(".sync-conflict") || tail.starts_with('_');
+    (marked && !digits.is_empty())
+        .then(|| digits.parse().ok())
+        .flatten()
+}
+
+/// Page content without the format version (that differs between app versions).
+fn content(mut data: PageData) -> PageData {
+    data.format_version = 0;
+    data
+}
+
+impl DiskWatch {
+    pub fn new(dir: PathBuf, now: Instant) -> Self {
+        Self {
+            known: scan(&dir),
+            dir,
+            ignored: HashSet::new(),
+            next_check: now + INTERVAL,
+        }
+    }
+
+    pub fn deadline(&self) -> Instant {
+        self.next_check
+    }
+
+    pub fn is_due(&self, now: Instant) -> bool {
+        now >= self.next_check
+    }
+
+    /// Skips the check that is due now (e.g. while drawing).
+    pub fn postpone(&mut self, now: Instant) {
+        self.next_check = now + INTERVAL;
+    }
+
+    /// Checks right away at the next opportunity (e.g. when the window gets the focus).
+    pub fn check_soon(&mut self, now: Instant) {
+        self.next_check = now;
+    }
+
+    /// Takes over what changed on disk. Call only while all local changes are saved.
+    pub fn check(
+        &mut self,
+        notebook: &mut Notebook,
+        manifest: &mut NotebookManifest,
+        now: Instant,
+    ) -> Outcome {
+        self.next_check = now + INTERVAL;
+        let mut outcome = Outcome::default();
+        let current = scan(&self.dir);
+        let changed = |name: &str| current.get(name) != self.known.get(name);
+        let mut retry: HashSet<String> = HashSet::new();
+
+        if changed(storage::OPEN_MARKER_FILE) {
+            outcome.opened_elsewhere = fs::read(self.dir.join(storage::OPEN_MARKER_FILE))
+                .ok()
+                .and_then(|b| serde_json::from_slice::<OpenMarker>(&b).ok())
+                .filter(|m| m.device != storage::device_name());
+        }
+
+        // Page order, titles, new and removed pages
+        let mut fresh_pages: HashSet<u64> = HashSet::new();
+        if changed("manifest.json") {
+            match storage::load_manifest(&self.dir) {
+                Ok(theirs) => match self.apply_manifest(notebook, manifest, theirs) {
+                    Ok(Some(new_ids)) => {
+                        fresh_pages = new_ids;
+                        outcome.structure_changed = true;
+                    }
+                    Ok(None) => {}
+                    Err(()) => {
+                        retry.insert("manifest.json".to_string());
+                    }
+                },
+                Err(_) => {
+                    retry.insert("manifest.json".to_string());
+                }
+            }
+        }
+
+        // Changed page files
+        for name in current.keys().filter(|n| changed(n)) {
+            let Some(id) = page_file_id(name) else {
+                continue;
+            };
+            let Some(page) = notebook.pages.iter_mut().find(|p| p.id == id) else {
+                continue;
+            };
+            if !page.is_loaded || fresh_pages.contains(&id) {
+                continue;
+            }
+            match storage::load_page(&self.dir, id) {
+                Ok(data) => {
+                    if content(PageData::from(&*page)) != content(data.clone()) {
+                        let mut reloaded = data.to_page();
+                        reloaded.title = reloaded.title.or_else(|| page.title.clone());
+                        *page = reloaded;
+                        outcome.reloaded_pages += 1;
+                    }
+                }
+                // Possibly still being written by the sync client: try again next time
+                Err(_) => {
+                    retry.insert(name.clone());
+                }
+            }
+        }
+
+        // Conflict copies of sync clients become pages of their own
+        let mut copies: Vec<&String> = current
+            .keys()
+            .filter(|n| !self.ignored.contains(*n))
+            .filter(|n| conflict_copy_of(n).is_some())
+            .collect();
+        copies.sort();
+        for name in copies {
+            let path = self.dir.join(name);
+            let data = fs::read(&path)
+                .ok()
+                .and_then(|b| serde_json::from_slice::<PageData>(&b).ok());
+            match data {
+                Some(data) => {
+                    let of = conflict_copy_of(name).unwrap_or(0);
+                    outcome
+                        .conflict_pages
+                        .push(add_conflict_page(notebook, data, of));
+                    // Keep the file, but don't take it over again
+                    let _ = fs::rename(&path, path.with_extension("json.imported"));
+                }
+                None => {
+                    self.ignored.insert(name.clone());
+                }
+            }
+        }
+        if !outcome.conflict_pages.is_empty() {
+            manifest.sync_from_notebook(notebook);
+        }
+
+        // Remember what we have seen, except files to look at again
+        let mut known = current;
+        for name in retry {
+            match self.known.get(&name) {
+                Some(old) => known.insert(name, *old),
+                None => known.remove(&name),
+            };
+        }
+        self.known = known;
+        outcome
+    }
+
+    /// Takes over the page list of a manifest written elsewhere. Returns the ids of pages
+    /// that are now unloaded placeholders, or `None` if nothing about the pages changed.
+    /// Fails (to be tried again) while the page to show can't be read yet.
+    fn apply_manifest(
+        &self,
+        notebook: &mut Notebook,
+        manifest: &mut NotebookManifest,
+        theirs: NotebookManifest,
+    ) -> Result<Option<HashSet<u64>>, ()> {
+        let mut ours = manifest.clone();
+        ours.sync_from_notebook(notebook);
+        if ours.pages == theirs.pages && ours.title == theirs.title {
+            return Ok(None);
+        }
+        let current_id = notebook.current_page().id;
+
+        // The page shown afterwards must be loaded. Read it before changing anything: a file
+        // the sync client is still writing must not end up set aside as damaged.
+        let shown = theirs
+            .pages
+            .iter()
+            .find(|m| m.id == current_id)
+            .or(theirs.pages.first())
+            .map(|m| m.id);
+        let mut shown_data = None;
+        if let Some(id) = shown {
+            if !notebook.pages.iter().any(|p| p.id == id && p.is_loaded) {
+                match storage::load_page(&self.dir, id) {
+                    Ok(data) => shown_data = Some(data),
+                    // Never saved because still empty
+                    Err(storage::StorageError::PageNotFound(_)) => {}
+                    Err(_) => return Err(()),
+                }
+            }
+        }
+
+        let mut old: HashMap<u64, Page> = notebook.pages.drain(..).map(|p| (p.id, p)).collect();
+        let mut placeholders = HashSet::new();
+        for meta in &theirs.pages {
+            let background = meta.background.unwrap_or(PageBackground::Grid5mm);
+            let size = storage::page_size_from(meta.size);
+            let mut page = match old.remove(&meta.id) {
+                Some(mut page) => {
+                    page.background = background;
+                    page.size = size;
+                    page
+                }
+                None => {
+                    placeholders.insert(meta.id);
+                    Page::new_unloaded(meta.id, background).with_size(size)
+                }
+            };
+            page.title = meta.title.clone();
+            notebook.pages.push(page);
+        }
+        if notebook.pages.is_empty() {
+            notebook.pages.push(Page::new(1, PageBackground::Grid5mm));
+        }
+        notebook.current_page_index = notebook
+            .pages
+            .iter()
+            .position(|p| p.id == current_id)
+            .unwrap_or(0)
+            .min(notebook.pages.len() - 1);
+        notebook.next_page_id = notebook.next_page_id.max(theirs.next_page_id);
+        notebook.next_stroke_id = notebook.next_stroke_id.max(theirs.next_stroke_id);
+        let index = notebook.current_page_index;
+        if !notebook.pages[index].is_loaded {
+            let page = &mut notebook.pages[index];
+            let mut loaded = match shown_data {
+                Some(data) => data.to_page(),
+                None => Page::new(page.id, page.background).with_size(page.size),
+            };
+            loaded.title = loaded.title.or(page.title.take());
+            *page = loaded;
+            placeholders.remove(&page.id);
+        }
+        let layout = manifest.layout;
+        *manifest = theirs;
+        manifest.layout = manifest.layout.or(layout);
+        manifest.sync_from_notebook(notebook);
+        Ok(Some(placeholders))
+    }
+}
+
+/// Adds a conflict copy as a new page after page `of` (or at the end), with fresh ids.
+/// Returns the id of the new page.
+fn add_conflict_page(notebook: &mut Notebook, mut data: PageData, of: u64) -> u64 {
+    let id = notebook.next_page_id;
+    data.id = id;
+    notebook.next_page_id += 1;
+    for stroke in &mut data.strokes {
+        stroke.id = notebook.next_stroke_id;
+        notebook.next_stroke_id += 1;
+    }
+    let original = notebook.pages.iter().position(|p| p.id == of);
+    let title = original
+        .and_then(|i| notebook.pages[i].title.clone())
+        .or(data.title.take());
+    data.title = Some(match title {
+        Some(title) => crate::t!("Konfliktkopie: {title}", title = title).to_string(),
+        None => crate::t!("Konfliktkopie").to_string(),
+    });
+    let page = data.to_page();
+    let at = original.map_or(notebook.pages.len(), |i| i + 1);
+    notebook.pages.insert(at, page);
+    if at <= notebook.current_page_index {
+        notebook.current_page_index += 1;
+    }
+    id
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::document::{Stroke, StylusSample};
+
+    fn stroke(id: u64, y: f32) -> Stroke {
+        let points = (0..5)
+            .map(|i| StylusSample {
+                position: glam::Vec2::new(100.0 + 50.0 * i as f32, y),
+                pressure: 0.5,
+                timestamp: 0.0,
+            })
+            .collect();
+        Stroke::new(id, points, Vec::new(), [0.0, 0.0, 0.0, 1.0])
+    }
+
+    /// Saved notebook with one stroke on its only page.
+    fn saved_notebook(dir: &Path) -> (Notebook, NotebookManifest) {
+        let mut notebook = Notebook::new();
+        notebook.current_page_mut().add_stroke(stroke(1, 100.0));
+        notebook.next_stroke_id = 2;
+        let mut manifest = NotebookManifest::new("nb", "Mathe");
+        storage::save_notebook(dir, &notebook, &mut manifest).unwrap();
+        (notebook, manifest)
+    }
+
+    #[test]
+    fn test_conflict_copy_names() {
+        assert_eq!(
+            conflict_copy_of("page_3 (conflicted copy 2026-10-27).json"),
+            Some(3)
+        );
+        assert_eq!(conflict_copy_of("page_12 (1).json"), Some(12));
+        assert_eq!(
+            conflict_copy_of("page_3.sync-conflict-20261027-1200-ABC.json"),
+            Some(3)
+        );
+        assert_eq!(conflict_copy_of("page_3_conflict.json"), Some(3));
+        assert_eq!(conflict_copy_of("page_3.json"), None);
+        assert_eq!(conflict_copy_of("page_x (1).json"), None);
+        assert_eq!(conflict_copy_of("manifest (1).json"), None);
+        assert_eq!(conflict_copy_of("page_3.json.imported"), None);
+    }
+
+    #[test]
+    fn test_own_saves_are_not_reported() {
+        let dir = tempfile::tempdir().unwrap();
+        let (mut notebook, mut manifest) = saved_notebook(dir.path());
+        let now = Instant::now();
+        let mut watch = DiskWatch::new(dir.path().to_path_buf(), now);
+
+        notebook.current_page_mut().add_stroke(stroke(2, 200.0));
+        notebook.add_page(PageBackground::Blank);
+        storage::save_notebook(dir.path(), &notebook, &mut manifest).unwrap();
+
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert!(outcome.is_empty(), "{outcome:?}");
+        assert_eq!(notebook.pages.len(), 2);
+    }
+
+    #[test]
+    fn test_changes_from_another_device_are_taken_over() {
+        let dir = tempfile::tempdir().unwrap();
+        let (mut notebook, mut manifest) = saved_notebook(dir.path());
+        let now = Instant::now();
+        let mut watch = DiskWatch::new(dir.path().to_path_buf(), now);
+        assert!(!watch.is_due(now));
+
+        // The other device adds a stroke on page 1 and a second page with a title
+        let (mut theirs, mut their_manifest) = storage::load_notebook(dir.path()).unwrap();
+        theirs.current_page_mut().add_stroke(stroke(5, 300.0));
+        let index = theirs.add_page(PageBackground::Lined);
+        theirs.pages[index].add_stroke(stroke(6, 400.0));
+        theirs.pages[index].title = Some("Übung".to_string());
+        theirs.next_stroke_id = 7;
+        storage::save_notebook(dir.path(), &theirs, &mut their_manifest).unwrap();
+
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert_eq!(outcome.reloaded_pages, 1);
+        assert!(outcome.structure_changed);
+        assert!(outcome.conflict_pages.is_empty());
+        assert_eq!(notebook.pages.len(), 2);
+        assert_eq!(notebook.pages[0].strokes.len(), 2);
+        assert_eq!(notebook.pages[1].title.as_deref(), Some("Übung"));
+        assert_eq!(notebook.pages[1].background, PageBackground::Lined);
+        assert!(notebook.next_stroke_id >= 7);
+        assert_eq!(manifest.pages.len(), 2);
+
+        // The new page loads from disk when needed
+        storage::ensure_page_loaded(dir.path(), &mut notebook, 1).unwrap();
+        assert_eq!(notebook.pages[1].strokes.len(), 1);
+
+        // Nothing new: nothing to report
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert!(outcome.is_empty(), "{outcome:?}");
+    }
+
+    #[test]
+    fn test_page_still_being_synced_is_tried_again() {
+        let dir = tempfile::tempdir().unwrap();
+        let (mut notebook, mut manifest) = saved_notebook(dir.path());
+        let now = Instant::now();
+        let mut watch = DiskWatch::new(dir.path().to_path_buf(), now);
+
+        // Elsewhere page 1 was deleted and page 2 added; its file is still half written
+        let (mut theirs, mut their_manifest) = storage::load_notebook(dir.path()).unwrap();
+        let index = theirs.add_page(PageBackground::Blank);
+        theirs.pages[index].add_stroke(stroke(9, 100.0));
+        storage::save_notebook(dir.path(), &theirs, &mut their_manifest).unwrap();
+        their_manifest.pages.remove(0);
+        storage::save_manifest(dir.path(), &their_manifest).unwrap();
+        let page_file = dir.path().join("page_2.json");
+        let complete = fs::read(&page_file).unwrap();
+        fs::write(&page_file, &complete[..complete.len() / 2]).unwrap();
+
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert!(outcome.is_empty(), "{outcome:?}");
+        assert_eq!(notebook.pages.len(), 1);
+        assert_eq!(notebook.pages[0].id, 1);
+        assert!(page_file.exists(), "must not be set aside as damaged");
+
+        fs::write(&page_file, &complete).unwrap();
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert!(outcome.structure_changed);
+        assert_eq!(notebook.pages.len(), 1);
+        assert_eq!(notebook.current_page().id, 2);
+        assert!(notebook.current_page().is_loaded);
+        assert_eq!(notebook.current_page().strokes.len(), 1);
+    }
+
+    #[test]
+    fn test_conflict_copy_becomes_a_page() {
+        let dir = tempfile::tempdir().unwrap();
+        let (mut notebook, mut manifest) = saved_notebook(dir.path());
+        notebook.add_page(PageBackground::Blank);
+        notebook.current_page_index = 1;
+        storage::save_notebook(dir.path(), &notebook, &mut manifest).unwrap();
+        let now = Instant::now();
+        let mut watch = DiskWatch::new(dir.path().to_path_buf(), now);
+
+        let mut copy = PageData::from(&notebook.pages[0]);
+        copy.strokes.push(
+            PageData::from(&{
+                let mut page = Page::new(1, PageBackground::Blank);
+                page.add_stroke(stroke(1, 500.0));
+                page
+            })
+            .strokes[0]
+                .clone(),
+        );
+        let name = "page_1 (conflicted copy 2026-10-27).json";
+        fs::write(dir.path().join(name), serde_json::to_vec(&copy).unwrap()).unwrap();
+        fs::write(dir.path().join("page_2 (1).json"), b"{ kaputt").unwrap();
+
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert_eq!(outcome.conflict_pages.len(), 1);
+        assert_eq!(notebook.pages.len(), 3);
+        let added = &notebook.pages[1];
+        assert_eq!(added.id, outcome.conflict_pages[0]);
+        assert_eq!(added.strokes.len(), 2);
+        assert!(added.title.is_some());
+        // Fresh ids, no clash with the original page
+        assert!(added.strokes.iter().all(|s| s.id >= 2));
+        assert_eq!(notebook.pages[0].strokes.len(), 1);
+        // The current page stays the same
+        assert_eq!(notebook.current_page_index, 2);
+        assert_eq!(manifest.pages.len(), 3);
+        assert!(!dir.path().join(name).exists());
+        assert!(dir.path().join(format!("{name}.imported")).exists());
+
+        // Taken over only once; the unreadable file is left alone
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert!(outcome.is_empty(), "{outcome:?}");
+        assert!(dir.path().join("page_2 (1).json").exists());
+    }
+
+    #[test]
+    fn test_open_marker() {
+        let dir = tempfile::tempdir().unwrap();
+        let (mut notebook, mut manifest) = saved_notebook(dir.path());
+        let marker_path = dir.path().join(storage::OPEN_MARKER_FILE);
+        let foreign = OpenMarker {
+            device: "tablet-of-pia".to_string(),
+            since: 1,
+        };
+        fs::write(&marker_path, serde_json::to_vec(&foreign).unwrap()).unwrap();
+
+        // Opening warns about the other device and puts our own marker in place
+        let lock = storage::NotebookLock::acquire(dir.path()).unwrap();
+        assert_eq!(lock.open_elsewhere.as_ref(), Some(&foreign));
+        let ours: OpenMarker = serde_json::from_slice(&fs::read(&marker_path).unwrap()).unwrap();
+        assert_eq!(ours.device, storage::device_name());
+
+        // The other device opens it again
+        let now = Instant::now();
+        let mut watch = DiskWatch::new(dir.path().to_path_buf(), now);
+        fs::write(&marker_path, serde_json::to_vec(&foreign).unwrap()).unwrap();
+        let outcome = watch.check(&mut notebook, &mut manifest, now);
+        assert_eq!(outcome.opened_elsewhere, Some(foreign));
+
+        // A foreign marker is not removed on closing
+        drop(lock);
+        assert!(marker_path.exists());
+        let lock = storage::NotebookLock::acquire(dir.path()).unwrap();
+        drop(lock);
+        assert!(!marker_path.exists());
+    }
+}

src/file_picker.rs

@@ -24,6 +24,8 @@ pub enum PickerKind {
     Image,
     /// PDF, Xournal++, Rnote or Saber file to import as a new notebook
     Import,
+    /// Folder for the notebooks (Issue #49): lists folders only
+    Folder,
 }
 
 impl PickerKind {
@@ -32,6 +34,7 @@ impl PickerKind {
         match self {
             PickerKind::Image => is_supported_image_path(path),
             PickerKind::Import => crate::import::is_importable_path(path),
+            PickerKind::Folder => false,
         }
     }
 }
@@ -64,6 +67,7 @@ impl FilePicker {
                 let preferred = match kind {
                     PickerKind::Image => tn!("🖼 Bilder"),
                     PickerKind::Import => tn!("⬇ Downloads"),
+                    PickerKind::Folder => tn!("🏠 Persönlicher Ordner"),
                 };
                 let all = shortcuts();
                 all.iter()
@@ -225,6 +229,7 @@ mod tests {
                 "Uni.sbn2"
             ]
         );
+        assert_eq!(names(PickerKind::Folder), ["aOrdner", "zOrdner"]);
     }
 
     #[test]

src/i18n.rs

@@ -151,6 +151,7 @@ mod tests {
         ("assets.rs", include_str!("assets.rs")),
         ("autosave.rs", include_str!("autosave.rs")),
         ("background.rs", include_str!("background.rs")),
+        ("disk_watch.rs", include_str!("disk_watch.rs")),
         ("document.rs", include_str!("document.rs")),
         ("export.rs", include_str!("export.rs")),
         ("file_picker.rs", include_str!("file_picker.rs")),

src/i18n/en.rs

@@ -464,4 +464,24 @@ pub const EN: &[(&str, &str)] = &[
     ("📄  Importieren (PDF, Xournal++, Rnote, Saber) …", "📄  Import (PDF, Xournal++, Rnote, Saber) …"),
     ("Keine Saber-Datei", "Not a Saber file"),
     ("Saber-Datei zu alt: bitte in einem aktuellen Saber öffnen und neu speichern", "Saber file too old: please open it in a current Saber and save it again"),
+    ("Die Notizbücher liegen bereits dort", "The notebooks are already there"),
+    ("Konfliktkopien als eigene Seiten übernommen: {count}", "Conflict copies added as separate pages: {count}"),
+    ("Speicherort geändert, {count} Notizbücher kopiert", "Storage location changed, {count} notebooks copied"),
+    ("Speicherort geändert: {path}", "Storage location changed: {path}"),
+    ("Speicherort nicht nutzbar: {err}", "Storage location not usable: {err}"),
+    ("Änderungen von einem anderen Gerät übernommen", "Changes from another device taken over"),
+    ("⚠ Auch auf '{device}' geöffnet – Änderungen dort können deine überschreiben", "⚠ Also open on '{device}' – changes there may overwrite yours"),
+    ("⚠ Gerade auf '{device}' geöffnet – Änderungen dort können deine überschreiben", "⚠ Just opened on '{device}' – changes there may overwrite yours"),
+    ("Konfliktkopie", "Conflict copy"),
+    ("Konfliktkopie: {title}", "Conflict copy: {title}"),
+    ("Die Notizbücher liegen dann im Unterordner „ink-notes“ des gewählten Ordners, z. B. im Nextcloud- oder Google-Drive-Ordner. Ist er noch leer, werden die vorhandenen Notizbücher dorthin kopiert; die alten bleiben erhalten.", "The notebooks are then kept in the subfolder “ink-notes” of the chosen folder, e.g. in your Nextcloud or Google Drive folder. If it is still empty, the existing notebooks are copied there; the old ones are kept."),
+    ("Keine Unterordner", "No subfolders"),
+    ("SPEICHERORT", "STORAGE LOCATION"),
+    ("Speicherort wählen", "Choose storage location"),
+    ("Standard", "Default"),
+    ("Z. B. einen Nextcloud- oder Google-Drive-Ordner wählen, um die Notizbücher mit anderen Geräten abzugleichen", "E.g. choose a Nextcloud or Google Drive folder to sync the notebooks with other devices"),
+    ("Zurück zum Standard-Speicherort", "Back to the default storage location"),
+    ("✔  Diesen Ordner wählen", "✔  Choose this folder"),
+    ("📁  Ändern …", "📁  Change …"),
+    ("📁 Speicherort wählen", "📁 Choose storage location"),
 ];

src/lib.rs

@@ -5,6 +5,7 @@ pub mod assets;
 pub mod autosave;
 pub mod background;
 pub mod canvas;
+pub mod disk_watch;
 pub mod document;
 pub mod export;
 pub mod file_picker;

src/settings.rs

@@ -229,6 +229,9 @@ pub struct UiSettings {
     pub language: Option<Language>,
     /// Eraser radius in mm (Issue #38)
     pub eraser_radius_mm: f32,
+    /// Folder of the notebooks, e.g. inside a Nextcloud folder; default location if unset
+    /// (Issue #49)
+    pub storage_dir: Option<PathBuf>,
 }
 
 impl Default for UiSettings {
@@ -249,6 +252,7 @@ impl Default for UiSettings {
             stylus_buttons: Vec::new(),
             language: None,
             eraser_radius_mm: 3.0,
+            storage_dir: None,
         }
     }
 }
@@ -349,6 +353,7 @@ mod tests {
             ],
             language: Some(Language::English),
             eraser_radius_mm: 6.0,
+            storage_dir: Some(PathBuf::from("/home/pia/Nextcloud/ink-notes")),
         };
         settings.save_to(&path).unwrap();
         assert_eq!(UiSettings::load_from(&path), settings);

src/storage.rs

@@ -939,6 +939,59 @@ pub fn ensure_page_loaded(
     }
 }
 
+/// Notebook folder inside a folder the user picked as storage location (Issue #49), e.g. the
+/// Nextcloud folder: `<chosen>/ink-notes`, unless the folder already is called `ink-notes`.
+pub fn storage_dir_in(chosen: &Path) -> PathBuf {
+    if chosen.file_name().is_some_and(|n| n == "ink-notes") {
+        chosen.to_path_buf()
+    } else {
+        chosen.join("ink-notes")
+    }
+}
+
+/// Prepares `to` as new storage location. If it holds no notebooks yet, the notebooks and
+/// `library.json` of `from` are copied there (lock files and open markers excluded); the
+/// originals stay where they are. Returns the number of copied notebooks.
+pub fn copy_storage(from: &Path, to: &Path) -> io::Result<usize> {
+    create_secure_dir_all(to)?;
+    if !list_notebooks(to).is_empty() {
+        return Ok(0);
+    }
+    let mut copied = 0;
+    for summary in list_notebooks(from) {
+        let target = to.join(&summary.id);
+        if target.exists() {
+            continue;
+        }
+        copy_dir(&from.join(&summary.id), &target)?;
+        copied += 1;
+    }
+    let library = from.join("library.json");
+    if library.is_file() && !to.join("library.json").exists() {
+        fs::copy(&library, to.join("library.json"))?;
+    }
+    Ok(copied)
+}
+
+/// Copies a notebook folder recursively, without lock file and open marker.
+fn copy_dir(from: &Path, to: &Path) -> io::Result<()> {
+    create_secure_dir_all(to)?;
+    for entry in fs::read_dir(from)? {
+        let entry = entry?;
+        let name = entry.file_name();
+        if name == ".lock" || name == OPEN_MARKER_FILE {
+            continue;
+        }
+        let kind = entry.file_type()?;
+        if kind.is_dir() {
+            copy_dir(&entry.path(), &to.join(&name))?;
+        } else if kind.is_file() {
+            fs::copy(entry.path(), to.join(&name))?;
+        }
+    }
+    Ok(())
+}
+
 /// Gibt das Standard-Verzeichnis für Notizbücher zurück (gemäß XDG-Spezifikation).
 pub fn default_storage_dir() -> PathBuf {
     if let Ok(xdg) = std::env::var("XDG_DATA_HOME") {
@@ -984,6 +1037,49 @@ pub struct NotebookSummary {
 pub struct NotebookLock {
     _file: File,
     pub path: PathBuf,
+    /// Another device that has the notebook open according to its marker (Issue #49)
+    pub open_elsewhere: Option<OpenMarker>,
+    marker: Option<(PathBuf, OpenMarker)>,
+}
+
+/// Which device has a notebook open, and since when (Unix seconds). The `.lock` file only
+/// locks on this computer; this marker (`.opened_by.json`) travels with sync clients such as
+/// Nextcloud, so another device can warn before both edit at the same time (Issue #49).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct OpenMarker {
+    pub device: String,
+    pub since: u64,
+}
+
+/// File name of the [`OpenMarker`] in the notebook folder.
+pub const OPEN_MARKER_FILE: &str = ".opened_by.json";
+
+/// Name of this device (host name).
+pub fn device_name() -> String {
+    let from_file = |path: &str| fs::read_to_string(path).ok();
+    std::env::var("INK_NOTES_DEVICE")
+        .ok()
+        .or_else(|| from_file("/proc/sys/kernel/hostname"))
+        .or_else(|| from_file("/etc/hostname"))
+        .or_else(|| std::env::var("COMPUTERNAME").ok())
+        .or_else(|| std::env::var("HOSTNAME").ok())
+        .map(|name| name.trim().to_string())
+        .filter(|name| !name.is_empty())
+        .unwrap_or_else(|| "?".to_string())
+}
+
+impl Drop for NotebookLock {
+    fn drop(&mut self) {
+        // Remove our marker, unless another device has replaced it meanwhile
+        if let Some((path, ours)) = &self.marker {
+            let current = fs::read(path)
+                .ok()
+                .and_then(|b| serde_json::from_slice::<OpenMarker>(&b).ok());
+            if current.as_ref() == Some(ours) {
+                let _ = fs::remove_file(path);
+            }
+        }
+    }
 }
 
 impl NotebookLock {
@@ -1007,10 +1103,31 @@ impl NotebookLock {
 
         use fs2::FileExt;
         match file.try_lock_exclusive() {
-            Ok(()) => Ok(Self {
+            Ok(()) => {
+                let device = device_name();
+                let marker_path = notebook_dir.join(OPEN_MARKER_FILE);
+                let open_elsewhere = fs::read(&marker_path)
+                    .ok()
+                    .and_then(|b| serde_json::from_slice::<OpenMarker>(&b).ok())
+                    .filter(|m| m.device != device);
+                let ours = OpenMarker {
+                    device,
+                    since: std::time::SystemTime::now()
+                        .duration_since(std::time::UNIX_EPOCH)
+                        .unwrap_or_default()
+                        .as_secs(),
+                };
+                let marker = serde_json::to_vec(&ours)
+                    .ok()
+                    .filter(|json| atomic_write_file(&marker_path, json).is_ok())
+                    .map(|_| (marker_path, ours));
+                Ok(Self {
                     _file: file,
                     path: lock_path,
-            }),
+                    open_elsewhere,
+                    marker,
+                })
+            }
             Err(e) => {
                 let kind = e.kind();
                 if kind == io::ErrorKind::WouldBlock || e.raw_os_error() == Some(11) {
@@ -2159,6 +2276,37 @@ mod tests {
         assert_eq!(damaged_summary.unwrap().status, NotebookStatus::Damaged);
     }
 
+    #[test]
+    fn test_copy_storage_to_new_location() {
+        let old = tempfile::tempdir().unwrap();
+        let cloud = tempfile::tempdir().unwrap();
+        let (notebook, mut manifest, dir) = create_new_notebook(old.path(), "Mathe").unwrap();
+        save_notebook(&dir, &notebook, &mut manifest).unwrap();
+        let _lock = NotebookLock::acquire(&dir).unwrap();
+        fs::create_dir_all(dir.join("assets")).unwrap();
+        fs::write(dir.join("assets").join("bild.png"), b"png").unwrap();
+        fs::write(old.path().join("library.json"), b"{}").unwrap();
+
+        assert_eq!(storage_dir_in(cloud.path()), cloud.path().join("ink-notes"));
+        let target = storage_dir_in(cloud.path());
+        assert_eq!(storage_dir_in(&target), target);
+
+        assert_eq!(copy_storage(old.path(), &target).unwrap(), 1);
+        let copied = target.join(&manifest.id);
+        assert!(load_notebook(&copied).is_ok());
+        assert!(copied.join("assets").join("bild.png").exists());
+        assert!(target.join("library.json").exists());
+        assert!(!copied.join(".lock").exists());
+        assert!(!copied.join(OPEN_MARKER_FILE).exists());
+        // The originals stay
+        assert!(load_notebook(&dir).is_ok());
+
+        // Notebooks already there (e.g. from another device): nothing is copied
+        create_new_notebook(old.path(), "Physik").unwrap();
+        assert_eq!(copy_storage(old.path(), &target).unwrap(), 0);
+        assert_eq!(list_notebooks(&target).len(), 1);
+    }
+
     #[test]
     fn test_notebook_instance_lock_prevents_concurrent_open() {
         let temp_dir = tempfile::tempdir().unwrap();

src/ui.rs

@@ -339,6 +339,8 @@ pub enum UiAction {
     InsertImageFile(std::path::PathBuf),
     /// PDF als neues Notizbuch importieren
     ImportFile(std::path::PathBuf),
+    /// Store the notebooks in this folder, `None` for the default location (Issue #49)
+    SetStorageDir(Option<std::path::PathBuf>),
     /// Aktuelle Seite als Bild oder SVG exportieren
     ExportPage(PageExportKind),
     /// Titel einer Seite setzen (leer = entfernen)
@@ -503,6 +505,9 @@ pub struct UiState {
     pub queued_actions: Vec<UiAction>,
     /// Zuletzt in der Dateiauswahl benutzter Ordner (während der Sitzung)
     pub last_picker_dir: Option<std::path::PathBuf>,
+    /// Folder holding the notebooks and whether the user picked it (Issue #49)
+    pub storage_dir: std::path::PathBuf,
+    pub storage_dir_custom: bool,
 }
 
 impl Default for UiState {
@@ -588,6 +593,8 @@ impl Default for UiState {
             page_title_for: None,
             page_picker_list: false,
             last_picker_dir: None,
+            storage_dir: std::path::PathBuf::new(),
+            storage_dir_custom: false,
         }
     }
 }
@@ -674,6 +681,12 @@ impl UiState {
         self.open_file_picker(PickerKind::Import);
     }
 
+    /// Opens the folder picker for the storage location in the home folder.
+    pub fn open_storage_picker(&mut self) {
+        self.close_overlays();
+        self.file_picker = Some(FilePicker::open(PickerKind::Folder, None));
+    }
+
     fn open_file_picker(&mut self, kind: PickerKind) {
         let dir = self.last_picker_dir.clone();
         self.close_overlays();
@@ -2839,6 +2852,17 @@ fn file_picker_dialog(ctx: &egui::Context, state: &mut UiState, actions: &mut Ve
              Texte und Bilder werden übernommen, bei Xournal++ und Saber auch PDF-Hintergründe."
             ),
         ),
+        PickerKind::Folder => (
+            t!("Speicherort wählen"),
+            t!("📁 Speicherort wählen"),
+            "📁",
+            t!("Keine Unterordner"),
+            t!(
+                "Die Notizbücher liegen dann im Unterordner „ink-notes“ des gewählten Ordners, \
+             z. B. im Nextcloud- oder Google-Drive-Ordner. Ist er noch leer, werden die \
+             vorhandenen Notizbücher dorthin kopiert; die alten bleiben erhalten."
+            ),
+        ),
     };
     dialog_window(title, state, false).show(ctx, |ui| {
         ui.set_width(460.0);
@@ -2877,6 +2901,16 @@ fn file_picker_dialog(ctx: &egui::Context, state: &mut UiState, actions: &mut Ve
         if let Some(err) = &picker.error {
             ui.colored_label(egui::Color32::from_rgb(255, 120, 120), err);
         }
+        if picker_kind == PickerKind::Folder
+            && ui
+                .add_sized(
+                    [ui.available_width(), 30.0],
+                    egui::Button::new(t!("✔  Diesen Ordner wählen")),
+                )
+                .clicked()
+        {
+            chosen = Some(picker.dir.clone());
+        }
         ui.add_space(4.0);
 
         egui::ScrollArea::vertical()
@@ -2923,10 +2957,13 @@ fn file_picker_dialog(ctx: &egui::Context, state: &mut UiState, actions: &mut Ve
         }
     }
     if let Some(path) = chosen {
+        if picker_kind != PickerKind::Folder {
             state.last_picker_dir = state.file_picker.as_ref().map(|p| p.dir.clone());
+        }
         actions.push(match picker_kind {
             PickerKind::Image => UiAction::InsertImageFile(path),
             PickerKind::Import => UiAction::ImportFile(path),
+            PickerKind::Folder => UiAction::SetStorageDir(Some(path)),
         });
         close = true;
     }
@@ -4397,6 +4434,36 @@ pub fn render_ui(
                                 }
                             });
 
+                            section(ui, t!("SPEICHERORT"));
+                            ui.add(
+                                egui::Label::new(
+                                    egui::RichText::new(state.storage_dir.display().to_string())
+                                        .size(11.5)
+                                        .color(egui::Color32::from_gray(190)),
+                                )
+                                .truncate(true),
+                            )
+                            .on_hover_text(state.storage_dir.display().to_string());
+                            ui.horizontal(|ui| {
+                                if ui
+                                    .add_sized([115.0, 28.0], egui::Button::new(t!("📁  Ändern …")))
+                                    .on_hover_text(t!("Z. B. einen Nextcloud- oder Google-Drive-Ordner wählen, um die Notizbücher mit anderen Geräten abzugleichen"))
+                                    .clicked()
+                                {
+                                    state.open_storage_picker();
+                                }
+                                if ui
+                                    .add_enabled(
+                                        state.storage_dir_custom,
+                                        egui::Button::new(t!("Standard")).min_size(egui::vec2(115.0, 28.0)),
+                                    )
+                                    .on_hover_text(t!("Zurück zum Standard-Speicherort"))
+                                    .clicked()
+                                {
+                                    actions.push(UiAction::SetStorageDir(None));
+                                }
+                            });
+
                             section(ui, t!("STATUS"));
                             let (dot_color, status_text) = autosave_status_style(autosave_status);
                             ui.horizontal(|ui| {