feat(player): resume last session on launch (#54)

The #52 teardown clears the session when idle/dismissed, so the
headset / lock-screen play button had nothing to resume and the user
lost their place.

- track.dart: toJson() (round-trips fromJson)
- db.dart: CachedResumeState single-row snapshot, schema 9->10 + migration
- audio_handler.dart: queuedTracks getter
- player_provider.dart: restoreQueue() — configure + setQueueFromTracks
  + seek, no play (restores PAUSED)
- resume_controller.dart: restores last snapshot paused on launch;
  persists {source,index,position_ms,tracks} debounced on track
  change / pause and immediately on app teardown; never clobbers the
  saved snapshot when the queue is empty so a teardown stays
  recoverable; restore gated on auth + no active session
- app.dart: wired into postFrame

db.g.dart regenerated by CI per project convention. Deferred fast-follow:
media-button-when-fully-stopped re-init (needs a configure()-injected
callback; tracked on #54).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/player/audio_handler.dart

@@ -127,6 +127,11 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   String? _queueSource;
   String? get queueSource => _queueSource;
 
+  /// The full TrackRef list backing the current queue (even when only
+  /// part is built as just_audio sources). Empty when nothing is
+  /// queued. Exposed for resume-state persistence (#54).
+  List<TrackRef> get queuedTracks => _lastTracks;
+
   /// Volume stream for UI subscribers. Mirrors the just_audio player's
   /// volume directly; set via setVolume(double).
   Stream<double> get volumeStream => _player.volumeStream;

flutter_client/lib/player/player_provider.dart

@@ -101,6 +101,36 @@ class PlayerActions {
     await h.play();
   }
 
+  /// Rebuilds a previously-persisted queue WITHOUT auto-playing, then
+  /// seeks to [position]. The resume-on-launch path (#54): the user
+  /// sees their last track in the mini bar, paused, and continues with
+  /// play / a media button. Mirrors playTracks' configure step so
+  /// streaming works after restore; intentionally no h.play().
+  Future<void> restoreQueue(
+    List<TrackRef> tracks, {
+    int initialIndex = 0,
+    Duration position = Duration.zero,
+    String? source,
+  }) async {
+    if (tracks.isEmpty) return;
+    final url   = await _ref.read(serverUrlProvider.future);
+    final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
+    final cache = _ref.read(albumCoverCacheProvider);
+    final audioCache = _ref.read(audioCacheManagerProvider);
+    final h = _ref.read(audioHandlerProvider)
+      ..configure(
+        baseUrl: url ?? '',
+        token: token,
+        coverCache: cache,
+        audioCacheManager: audioCache,
+        likeBridge: _buildLikeBridge(),
+      );
+    await h.setQueueFromTracks(tracks, initialIndex: initialIndex, source: source);
+    if (position > Duration.zero) {
+      await h.seek(position);
+    }
+  }
+
   /// Builds the adapter the audio handler uses to read + flip the
   /// current track's like state in response to external media-
   /// controller events (Wear's heart button, lock-screen favorite).