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>

native-route/linux/rssuper-sync.timer

@@ -0,0 +1,23 @@
+[Unit]
+Description=RSSuper Background Sync Timer
+Documentation=man:rssuper(1)
+
+[Timer]
+# On-boot delay (randomized between 1-5 minutes)
+OnBootSec=1min
+RandomizedDelaySec=4min
+
+# On-unit-active delay (6 hours after service starts)
+OnUnitActiveSec=6h
+
+# Accuracy (allow ±15 minutes)
+AccuracySec=15min
+
+# Persist timer across reboots
+Persistent=true
+
+# Wake system if sleeping to run timer
+WakeSystem=true
+
+[Install]
+WantedBy=timers.target