feat(audio): looping per-season background music system

MusicService (looping audioplayers player, independent of SFX) driven by the
active season theme's new 'bgm' key; switches track on season change, pauses on
app background, all failures swallowed. Separate Music on/off toggle in Settings
(persisted, independent of SFX). Season packs carry bgm keys (menu/season_001/
season_002), manifest regenerated. Assets slot assets/audio/bgm/ ready — drop in
menu.mp3/season_001.mp3/season_002.mp3 (CC0) and it plays; silent until then.
180 tests green, analyze clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/app.dart

@@ -13,21 +13,44 @@ class BlockSeasonsApp extends ConsumerStatefulWidget {
   ConsumerState<BlockSeasonsApp> createState() => _BlockSeasonsAppState();
 }
 
-class _BlockSeasonsAppState extends ConsumerState<BlockSeasonsApp> {
+class _BlockSeasonsAppState extends ConsumerState<BlockSeasonsApp>
+    with WidgetsBindingObserver {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addObserver(this);
     WidgetsBinding.instance.addPostFrameCallback((_) {
       ref.read(consentServiceProvider).ensureConsentAndInitialize();
       // Eagerly start the IAP service so its purchase stream is live for the
       // whole session — restores and interrupted/deferred transactions are
       // delivered (and completed) even if the player never opens Settings.
       ref.read(iapServiceProvider);
+      // Start background music for the current context (menu by default).
+      ref.read(musicServiceProvider).playKey(ref.read(activeThemeProvider).bgm);
     });
   }
 
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    // Pause music while the app is not in the foreground.
+    ref
+        .read(musicServiceProvider)
+        .setBackgrounded(state != AppLifecycleState.resumed);
+  }
+
   @override
   Widget build(BuildContext context) {
+    // Switch the looping track whenever the active season theme changes
+    // (home/menu -> a season -> back). playKey is a no-op if unchanged.
+    ref.listen(activeThemeProvider, (_, next) {
+      ref.read(musicServiceProvider).playKey(next.bgm);
+    });
     return MaterialApp(
       onGenerateTitle: (context) => AppLocalizations.of(context)!.appTitle,
       debugShowCheckedModeBanner: false,