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-manager-tests.vala

@@ -0,0 +1,82 @@
+/*
+ * NotificationManagerTests.vala
+ * 
+ * Unit tests for Linux notification manager.
+ */
+
+using Gio;
+using GLib;
+using Gtk;
+
+public class RSSuper.NotificationManagerTests {
+    
+    public static int main(string[] args) {
+        Test.init(ref args);
+        
+        Test.add_func("/notification-manager/instance", () => {
+            var manager = NotificationManager.get_instance();
+            assert(manager != null);
+        });
+        
+        Test.add_func("/notification-manager/initialize", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            assert(manager.get_badge() != null);
+        });
+        
+        Test.add_func("/notification-manager/set-unread-count", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.set_unread_count(5);
+            assert(manager.get_unread_count() == 5);
+        });
+        
+        Test.add_func("/notification-manager/clear-unread-count", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.set_unread_count(5);
+            manager.clear_unread_count();
+            assert(manager.get_unread_count() == 0);
+        });
+        
+        Test.add_func("/notification-manager/badge-visibility", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.set_badge_visibility(true);
+            assert(manager.should_show_badge() == false);
+            
+            manager.set_unread_count(1);
+            assert(manager.should_show_badge() == true);
+        });
+        
+        Test.add_func("/notification-manager/show-badge", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.show_badge();
+            assert(manager.get_badge() != null);
+        });
+        
+        Test.add_func("/notification-manager/hide-badge", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.hide_badge();
+            var badge = manager.get_badge();
+            assert(badge != null);
+        });
+        
+        Test.add_func("/notification-manager/show-badge-with-count", () => {
+            var manager = NotificationManager.get_instance();
+            manager.initialize();
+            
+            manager.show_badge_with_count(10);
+            assert(manager.get_badge() != null);
+        });
+        
+        return Test.run();
+    }
+}