feat: Implement force sync functionality in settings screen and update initial sync logic

lib/app.dart

@@ -202,8 +202,9 @@ class _NotesAppState extends State<NotesApp>
 
       // Start periodic sync
       _startPeriodicSync();
-      // Run an initial full sync immediately to pull server changes
-      unawaited(_performSync(forceFull: true));
+      // Run an initial sync immediately and let the repository use the
+      // stored lastSyncAt when it exists.
+      unawaited(_performSync());
     } catch (e) {
       // If the database file is not a valid SQLite DB (e.g., wrong key or corruption),
       // reset the local vault so the app doesn't crash. The reset will delete DB files
@@ -720,6 +721,7 @@ class _NotesAppState extends State<NotesApp>
             key: const ValueKey<String>('settings-screen'),
             onDeleteAllData: _resetLocalVaultData,
             onBackToHome: _openHome,
+            onForceSync: () => _performSync(forceFull: true),
           );
 
     return MaterialApp(