* Implemented folder-based annotation synchronization and legacy book ID migration.
- Added `syncLocalAnnotationsToFolder` and `importAnnotationBundle` to `RecentFilesRepository` to handle bundling ink, rich text, layouts, and text boxes into a single JSON sidecar file.
- Introduced `saveAnnotationSidecar` and `getAnnotationSidecar` in `LocalSyncUtils` to manage hidden synchronization files in linked folders using SAF.
- Updated `MainViewModel` to trigger annotation syncing on book close and added a migration utility for legacy book IDs.
- Enhanced `FolderSyncWorker` to include a dedicated phase for importing newer annotation bundles from synchronized folders.
- Fixed an ID mismatch edge case in `PdfViewerScreen` by initiating data migration when a legacy ID is detected.
- Added `selectedBookId` to `ReaderScreenState` to better track the active document across the application.
* fix(sync): resolve Syncthing file lock conflicts and annotation migration bug
- Implement atomic saves (write-to-temp then rename) for annotation sidecars in `LocalSyncUtils` to prevent `.tmp` file locking and partial writes when using third-party sync tools like Syncthing.
- Add robust conflict resolution for annotation sidecars (`resolveAndCleanAnnotationConflicts`) to automatically detect, keep the newest version, and clean up `.sync-conflict` files.
- Fix a bug in `MainViewModel.checkAndMigrateLegacyBookId` where older legacy local files would blindly overwrite newly synced annotation files by adding a timestamp-based clobber check.
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