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.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")),