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/gsettings/org.rssuper.sync.gschema.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<schemalist>
+  <schema id="org.rssuper.sync" path="/org/rssuper/sync/">
+    <key type="t" name="last-sync-timestamp">
+      <default>0</default>
+      <summary>Last sync timestamp</summary>
+      <description>The Unix timestamp of the last successful sync</description>
+    </key>
+    <key type="i" name="preferred-sync-interval">
+      <default>21600</default>
+      <summary>Preferred sync interval in seconds</summary>
+      <description>The preferred interval between sync operations (default: 6 hours)</description>
+    </key>
+    <key type="b" name="auto-sync-enabled">
+      <default>true</default>
+      <summary>Auto-sync enabled</summary>
+      <description>Whether automatic background sync is enabled</description>
+    </key>
+    <key type="i" name="sync-on-wifi-only">
+      <default>0</default>
+      <summary>Sync on Wi-Fi only</summary>
+      <description>0=always, 1=Wi-Fi only, 2=never</description>
+    </key>
+  </schema>
+</schemalist>