feat: implement cross-platform features and UI integration

- iOS: Add BackgroundSyncService, SyncScheduler, SyncWorker, BookmarkViewModel, FeedViewModel
- iOS: Add BackgroundSyncService, SyncScheduler, SyncWorker services
- Linux: Add settings-store.vala, State.vala signals, view widgets (FeedList, FeedDetail, AddFeed, Search, Settings, Bookmark)
- Linux: Add bookmark-store.vala, bookmark vala model, search-service.vala
- Android: Add NotificationService, NotificationManager, NotificationPreferencesStore
- Android: Add BookmarkDao, BookmarkRepository, SettingsStore
- Add unit tests for iOS, Android, Linux
- Add integration tests
- Add performance benchmarks
- Update tasks and documentation

Co-Authored-By: Paperclip <noreply@paperclip.ing>

linux/src/repository/bookmark-repository.vala

@@ -0,0 +1,24 @@
+/*
+ * BookmarkRepository.vala
+ * 
+ * Repository for bookmark operations.
+ */
+
+namespace RSSuper {
+
+    /**
+     * BookmarkRepository - Interface for bookmark repository operations
+     */
+    public interface BookmarkRepository : Object {
+        public abstract void get_all_bookmarks(State<Bookmark[]> callback);
+        public abstract Bookmark? get_bookmark_by_id(string id) throws Error;
+        public abstract Bookmark? get_bookmark_by_feed_item_id(string feed_item_id) throws Error;
+        public abstract void add_bookmark(Bookmark bookmark) throws Error;
+        public abstract void update_bookmark(Bookmark bookmark) throws Error;
+        public abstract void delete_bookmark(string id) throws Error;
+        public abstract void delete_bookmark_by_feed_item_id(string feed_item_id) throws Error;
+        public abstract int get_bookmark_count() throws Error;
+        public abstract Bookmark[] get_bookmarks_by_tag(string tag) throws Error;
+    }
+
+}