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/data/save_repository.dart

@@ -45,6 +45,9 @@ class SaveRepository {
       _soundEnabled =
           (json['flags'] as Map<String, dynamic>?)?['soundEnabled'] as bool? ??
               true;
+      _musicEnabled =
+          (json['flags'] as Map<String, dynamic>?)?['musicEnabled'] as bool? ??
+              true;
     }
   }
 
@@ -60,12 +63,14 @@ class SaveRepository {
   int _endlessBest = 0;
   bool _adsRemoved = false;
   bool _soundEnabled = true;
+  bool _musicEnabled = true;
 
   StreakState get streak => _streak;
   bool get tutorialDone => _tutorialDone;
   int get endlessBest => _endlessBest;
   bool get adsRemoved => _adsRemoved;
   bool get soundEnabled => _soundEnabled;
+  bool get musicEnabled => _musicEnabled;
 
   Future<void> markTutorialDone() {
     _tutorialDone = true;
@@ -82,6 +87,11 @@ class SaveRepository {
     return _flush();
   }
 
+  Future<void> setMusicEnabled(bool value) {
+    _musicEnabled = value;
+    return _flush();
+  }
+
   Future<void> recordEndlessScore(int score) {
     if (score > _endlessBest) _endlessBest = score;
     return _flush();
@@ -154,6 +164,7 @@ class SaveRepository {
             'tutorialDone': _tutorialDone,
             'adsRemoved': _adsRemoved,
             'soundEnabled': _soundEnabled,
+            'musicEnabled': _musicEnabled,
           },
           'endless': {'best': _endlessBest},
         }),