feat: add auto-scan option for local library

Add a new 'Auto Scan' setting under Local Library with four modes:
off, every app open (10min cooldown), daily, and weekly. The app
uses WidgetsBindingObserver to trigger incremental scans on launch
and when resuming from background, respecting the configured
cooldown based on the last scan timestamp.

lib/providers/settings_provider.dart

@@ -518,6 +518,11 @@ class SettingsNotifier extends Notifier<AppSettings> {
     _saveSettings();
   }
 
+  void setLocalLibraryAutoScan(String mode) {
+    state = state.copyWith(localLibraryAutoScan: mode);
+    _saveSettings();
+  }
+
   void setTutorialComplete() {
     state = state.copyWith(hasCompletedTutorial: true);
     _saveSettings();