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/l10n/app_en.arb

@@ -60,5 +60,6 @@
   "restorePurchases": "Restore purchases",
   "adsRemovedThanks": "Ads removed — thank you!",
   "purchaseUnavailable": "Purchases are unavailable right now.",
-  "soundAndVibration": "Sound & vibration"
+  "soundAndVibration": "Sound & vibration",
+  "music": "Music"
 }