Add app reset (Réglages) and pull-to-refresh feeds

- Réglages sidebar screen with a destructive "Réinitialiser l'application"
  button (confirmation alert). Reset wipes the database, downloaded files, play
  state, queue, and sync settings (UserDefaults + Keychain), and stops playback.
- Pull-to-refresh: swipe down on the library re-fetches all subscriptions'
  feeds; on a podcast's screen it refreshes that feed (new episodes via the
  existing upsert/dedup).
- Supporting API: LibraryStore.deleteAllData, PlaybackController.stop,
  DownloadController.reset, SyncSettings.reset, KeychainStore.delete,
  AppModel.refreshAllFeeds/refresh/resetApplication. + reset test.

46 package tests green; iOS + macOS build; Réglages screen verified in simulator.

Co-Authored-By: Claude <claude@anthropic.com>