feat(android13): make All Files Access optional

- Android 13+ now only requires READ_MEDIA_AUDIO by default
- MANAGE_EXTERNAL_STORAGE is optional and can be enabled in Settings
- Added 'All Files Access' toggle in Download Settings (Android 13+ only)
- Users who encounter write errors can enable full storage access
- Respects privacy-conscious users who prefer limited permissions

lib/providers/settings_provider.dart

@@ -251,6 +251,11 @@ class SettingsNotifier extends Notifier<AppSettings> {
     state = state.copyWith(lossyBitrate: bitrate, lossyFormat: format);
     _saveSettings();
   }
+
+  void setUseAllFilesAccess(bool enabled) {
+    state = state.copyWith(useAllFilesAccess: enabled);
+    _saveSettings();
+  }
 }
 
 final settingsProvider = NotifierProvider<SettingsNotifier, AppSettings>(