feat(player): surface playback errors via debounced SnackBar (#58)

_handlePlaybackError silently skipped a dead track (404 / decoder /
EOS / network drop) with only a debugPrint, hiding the signal that
tells a broken track from a flaky app.

- audio_handler: _playbackErrors broadcast stream; emit the failing
  track title (mediaItem.value?.title — correctly mapped post-#49)
  before the skip/pause
- playback_error_reporter (new): global scaffoldMessengerKey +
  reporter that buffers, 2s-debounces, and coalesces bursts into one
  SnackBar ("Couldn't play X — skipping" / "Skipped N unplayable
  tracks")
- app.dart: scaffoldMessengerKey on MaterialApp.router + postFrame read

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

flutter_client/lib/player/audio_handler.dart

@@ -151,6 +151,14 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   Stream<Duration> get positionStream => _player.positionStream;
   Duration get position => _player.position;
 
+  /// Broadcasts the title of a track that just failed to play (404,
+  /// decoder failure, premature EOS, network drop) and was auto-skipped
+  /// or paused by _handlePlaybackError. The app listens and surfaces a
+  /// debounced/coalesced SnackBar so a silent skip isn't mysterious.
+  /// App-lifetime singleton handler — intentionally never closed.
+  final _playbackErrors = StreamController<String>.broadcast();
+  Stream<String> get playbackErrorStream => _playbackErrors.stream;
+
   void configure({
     required String baseUrl,
     required String? token,
@@ -478,6 +486,12 @@ class MinstrelAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandl
   /// If we're at the last track, seekToNext is a no-op; the state
   /// drops to idle and _broadcastState reflects that.
   Future<void> _handlePlaybackError() async {
+    // mediaItem is the correctly-mapped current track (post #49 logical
+    // index), so it's the one that just failed — no player-index math.
+    final failed = mediaItem.value?.title;
+    if (failed != null && failed.isNotEmpty) {
+      _playbackErrors.add(failed);
+    }
     final currentIdx = _player.currentIndex;
     final queueLen = queue.value.length;
     if (currentIdx == null || currentIdx + 1 >= queueLen) {