feat(player): restore playback session after process death

Queue, current index, position, and shuffle flag are snapshotted to a
new app_state playback_session table on pause, track change, and queue
mutation (never on position ticks). On launch the session is restored
paused into the mini player; the first play seeks to the saved
position. Plain file paths that no longer exist are dropped, an
explicit stop or mini-player dismiss clears the snapshot, and the
installation-restore guard wipes it alongside the download queue.

lib/screens/main_shell.dart

@@ -119,6 +119,7 @@ class _MainShellState extends ConsumerState<MainShell>
       );
       _initialSafRepairComplete = true;
       if (!mounted) return;
+      unawaited(restorePersistedPlaybackSession());
       _setupShareListener();
       await _checkSafMigration();
       final updateDialogShown = await _checkForUpdates();