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/ui/screens/settings_screen.dart

@@ -15,6 +15,7 @@ class SettingsScreen extends ConsumerWidget {
     final l10n = AppLocalizations.of(context)!;
     final adsRemoved = ref.watch(adsRemovedProvider);
     final soundOn = ref.watch(soundEnabledProvider);
+    final musicOn = ref.watch(musicEnabledProvider);
     final iap = ref.read(iapServiceProvider);
 
     ref.listen<bool>(adsRemovedProvider, (prev, next) {
@@ -44,6 +45,12 @@ class SettingsScreen extends ConsumerWidget {
                 onChanged: (v) =>
                     ref.read(soundEnabledProvider.notifier).set(v),
               ),
+              SwitchListTile(
+                title: Text(l10n.music),
+                value: musicOn,
+                onChanged: (v) =>
+                    ref.read(musicEnabledProvider.notifier).set(v),
+              ),
               const Divider(),
               ListTile(
                 title: Text(l10n.removeAds),