feat(player): resume on media-button when session torn down (#448)

Fast-follow of #54. After the #52 idle/dismiss teardown, pressing play
on a headset / watch / lock screen did nothing (handler.play() was just
_player.play() with nothing loaded; the handler is Riverpod-agnostic).

- audio_handler: _resumeHook + setResumeHook(); play() is now async —
  when mediaItem == null and a hook is set it awaits the hook (which
  restores + plays) and returns, else _player.play()
- resume_controller: extract shared _loadAndRestore() (bool); _restore()
  keeps the paused launch path; new resumeFromMediaButton() restores
  then starts playback; start() registers it via setResumeHook

Recursion-safe (post-restore mediaItem != null so the re-play hits
_player.play()); no-op when nothing to resume / auth missing / a
session is already active.

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

flutter_client/lib/player/audio_handler.dart

@@ -173,6 +173,13 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
     if (audioCacheManager != null) _audioCacheManager = audioCacheManager;
   }
 
+  /// Invoked by play() when nothing is loaded (mediaItem == null) so a
+  /// media-button press after the #52 teardown resumes the last
+  /// persisted session (#448). Registered by ResumeController.start();
+  /// null until then (then it's a no-op fall-through to _player.play()).
+  Future<void> Function()? _resumeHook;
+  void setResumeHook(Future<void> Function() hook) => _resumeHook = hook;
+
   Future<void> setQueueFromTracks(
     List<TrackRef> tracks, {
     int initialIndex = 0,
@@ -598,7 +605,18 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   }
 
   @override
-  Future<void> play() => _player.play();
+  Future<void> play() async {
+    // Nothing loaded (fresh, or torn down by the #52 idle/dismiss
+    // teardown): a media-button press resumes the last persisted
+    // session instead of no-op'ing (#448). The hook restores the queue
+    // and starts playback itself, so we return without calling
+    // _player.play() on an empty player.
+    if (mediaItem.value == null && _resumeHook != null) {
+      await _resumeHook!();
+      return;
+    }
+    await _player.play();
+  }
 
   @override
   Future<void> pause() => _player.pause();