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/app.dart

@@ -9,6 +9,7 @@ import 'cache/prefetcher.dart';
 import 'cache/resume_controller.dart';
 import 'cache/sync_controller.dart';
 import 'player/play_events_reporter.dart';
+import 'player/playback_error_reporter.dart';
 import 'shared/live_events_dispatcher.dart';
 import 'shared/routing.dart';
 import 'theme/theme_data.dart';
@@ -68,6 +69,10 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
       // pause, and app teardown. Pairs with the #52 idle teardown so a
       // torn-down session is recoverable instead of lost.
       ref.read(resumeControllerProvider);
+      // Playback-error reporter (#58): turns the handler's silent
+      // dead-track skips into a debounced/coalesced SnackBar so a
+      // vanished track isn't mysterious (and aids server/cache debug).
+      ref.read(playbackErrorReporterProvider);
       // Offline marker (#427 S1): periodic /healthz reachability
       // probe → offlineProvider. Read here to start the poller; S4
       // gates system-playlist play + Shuffle-all on it.
@@ -81,6 +86,7 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
     final mode = ref.watch(themeModeProvider).value ?? AppThemeMode.system;
     return MaterialApp.router(
       title: 'Minstrel',
+      scaffoldMessengerKey: scaffoldMessengerKey,
       theme: buildLightTheme(),
       darkTheme: buildDarkTheme(),
       themeMode: mode.materialMode,