feat(sync): built-in Nextcloud sync (Issue #49, stage 2)

Sign in to any Nextcloud server (also private ones in a subfolder, with
their own port or a self-signed certificate) through the browser with
Login Flow v2; the app gets an app password of its own, which
"Disconnect" revokes again.

A background thread syncs the storage folder with the folder ink-notes
on the server every 30 seconds and shortly after saving, three-way
against the state after the last pass: one-sided changes are copied,
manifests and library.json are merged, a page changed on both sides is
kept twice (the server's version as conflict copy, which disk_watch
turns into a page), empty pages give way, deletions go into the trash
on both sides and lose against changes. Changes to the open notebook
wait while it has unsaved changes. Folder ETags spare listing unchanged
notebooks; the pass stops instead of deleting when the storage folder
or the server folder is missing.

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

README.md

@@ -74,6 +74,7 @@ The interface is available in English and German; ink-notes follows the system l
 - **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.
+- **Nextcloud built in** (☰ menu, "Nextcloud"; also on Android and without a sync client): enter the address of your Nextcloud – also a private server, in a subfolder (`example.org/nextcloud`) or with its own port. Signing in happens in the browser (also with two-factor sign-in); the app gets an app password of its own, which you can revoke in Nextcloud under Settings → Security at any time, and never sees your real password. The notebooks are synced with the folder `ink-notes` in your Nextcloud files every 30 seconds and shortly after saving; the app keeps working offline. If a page was changed on two devices, both versions are kept (the other one as a page of its own); deleted notebooks and pages go into the trash (locally `.trash`, on the server its trash bin). A self-signed certificate is trusted once you have confirmed its fingerprint; addresses with `http://` only work with a warning, since the app password would be readable on the network. "Disconnect" stops syncing and revokes the app password; the notebooks stay. Use either the built-in sync or a sync client's folder as storage location, not both for the same notebooks.
 
 ### 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.
@@ -197,6 +198,8 @@ src/
 ├── history.rs      # Undo/redo
 ├── storage.rs      # Loading/saving (atomic), notebooks, trash, storage location
 ├── disk_watch.rs   # Taking over changes from other devices (sync clients)
+├── nextcloud.rs    # Nextcloud: sign-in in the browser, WebDAV, pinned certificates
+├── cloud_sync.rs   # Built-in sync with Nextcloud (comparison, merging, background thread)
 ├── library.rs      # Notebook folders (library.json)
 ├── math.rs         # Exact evaluation of handwritten math (Issue #46)
 ├── math_ink.rs     # Recognition of handwritten math ($P point clouds)