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/tests/notification-service-tests.vala

@@ -0,0 +1,75 @@
+/*
+ * NotificationServiceTests.vala
+ * 
+ * Unit tests for Linux notification service using Gio.Notification API.
+ */
+
+using Gio;
+using GLib;
+using Gtk;
+
+public class RSSuper.NotificationServiceTests {
+    
+    private NotificationService? _service;
+    
+    public static int main(string[] args) {
+        Test.init(ref args);
+        
+        Test.add_func("/notification-service/create", () => {
+            var service = new NotificationService();
+            assert(service != null);
+            assert(service.is_available());
+        });
+        
+        Test.add_func("/notification-service/create-with-params", () => {
+            var service = new NotificationService();
+            var notification = service.create("Test Title", "Test Body");
+            assert(notification != null);
+        });
+        
+        Test.add_func("/notification-service/create-with-icon", () => {
+            var service = new NotificationService();
+            var notification = service.create("Test Title", "Test Body", "icon-name");
+            assert(notification != null);
+        });
+        
+        Test.add_func("/notification-service/urgency-levels", () => {
+            var service = new NotificationService();
+            
+            var normal = service.create("Test", "Body", Urgency.NORMAL);
+            assert(normal != null);
+            
+            var low = service.create("Test", "Body", Urgency.LOW);
+            assert(low != null);
+            
+            var critical = service.create("Test", "Body", Urgency.CRITICAL);
+            assert(critical != null);
+        });
+        
+        Test.add_func("/notification-service/default-title", () => {
+            var service = new NotificationService();
+            var title = service.get_default_title();
+            assert(!string.IsNullOrEmpty(title));
+        });
+        
+        Test.add_func("/notification-service/default-urgency", () => {
+            var service = new NotificationService();
+            var urgency = service.get_default_urgency();
+            assert(urgency == Urgency.NORMAL);
+        });
+        
+        Test.add_func("/notification-service/set-default-title", () => {
+            var service = new NotificationService();
+            service.set_default_title("Custom Title");
+            assert(service.get_default_title() == "Custom Title");
+        });
+        
+        Test.add_func("/notification-service/set-default-urgency", () => {
+            var service = new NotificationService();
+            service.set_default_urgency(Urgency.CRITICAL);
+            assert(service.get_default_urgency() == Urgency.CRITICAL);
+        });
+        
+        return Test.run();
+    }
+}