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_ko.arb

@@ -32,5 +32,6 @@
   "restorePurchases": "구매 복원",
   "adsRemovedThanks": "광고가 제거되었습니다 — 감사합니다!",
   "purchaseUnavailable": "지금은 구매를 사용할 수 없습니다.",
-  "soundAndVibration": "소리 및 진동"
+  "soundAndVibration": "소리 및 진동",
+  "music": "음악"
 }