fix: replace raw setTimeout with tracked timers in game audio (BUG-4)

Added audioTimeout() to audio/context.ts — tracked timer set cleared on
scene destroy via clearAllAudioTimers(). Converted 20 sfx.ts + 3 voice.ts
raw setTimeout calls to audioTimeout. Converted FightScene playEntrance
timeout to trackedTimeout. Remaining setTimeout in tts.ts worker layer
and music.ts (already tracked via setMusicTimeout) are self-managing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/game/audio/context.ts

@@ -120,4 +120,18 @@ export function resetGainNodes() {
   }
 }
 
+// Tracked audio timers — SFX fire-and-forget delays that must be cleaned on scene destroy
+const _audioTimers = new Set<ReturnType<typeof setTimeout>>()
+
+export function audioTimeout(fn: () => void, ms: number): ReturnType<typeof setTimeout> {
+  const id = setTimeout(() => { _audioTimers.delete(id); fn() }, ms)
+  _audioTimers.add(id)
+  return id
+}
+
+export function clearAllAudioTimers() {
+  for (const id of _audioTimers) clearTimeout(id)
+  _audioTimers.clear()
+}
+
 export { MUSIC_VOL, SFX_VOL }