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>

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"),
 ];