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.service

@@ -0,0 +1,23 @@
+[Unit]
+Description=RSSuper Background Sync Service
+Documentation=man:rssuper(1)
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/opt/rssuper/bin/rssuper-sync
+StandardOutput=journal
+StandardError=journal
+
+# Security hardening
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=read-only
+PrivateTmp=yes
+
+# Timeout (5 minutes)
+TimeoutStartSec=300
+
+[Install]
+WantedBy=multi-user.target