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/state/State.vala

@@ -25,6 +25,9 @@ namespace RSSuper {
         private string? _message;
         private Error? _error;
 
+        public signal void state_changed();
+        public signal void data_changed();
+
         public State() {
             _state = State.IDLE;
         }
@@ -92,6 +95,7 @@ namespace RSSuper {
             _data = null;
             _message = null;
             _error = null;
+            state_changed();
         }
 
         public void set_success(T data) {
@@ -99,12 +103,15 @@ namespace RSSuper {
             _data = data;
             _message = null;
             _error = null;
+            state_changed();
+            data_changed();
         }
 
         public void set_error(string message, Error? error = null) {
             _state = State.ERROR;
             _message = message;
             _error = error;
+            state_changed();
         }
     }
 }