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>

content/manifest.json

@@ -7,13 +7,13 @@
       "seasonId": "season_001",
       "version": 1,
       "packUrl": "seasons/season_001/pack.json",
-      "sha256": "5b20b88251931838563aaaa7729f48e5a35f09dbf80c576b9bc2ec944050fc0a"
+      "sha256": "6018fc20187e5835e1a5bc5a3626479dcd448e3a9f35c708b89722587a881468"
     },
     {
       "seasonId": "season_002",
       "version": 1,
       "packUrl": "seasons/season_002/pack.json",
-      "sha256": "47cc115f9982ade7df686b28aa95a82edcc1e8a4aae5f13319e7131477855de3"
+      "sha256": "7be1d0082d9fa81b25938c340801baf9cc0deecdbbd0cdc2d75af443e9fb8552"
     }
   ]
 }