Adds per-zone battle music (battle-dark-cave-mix, battle-windy-peaks-mix)
with a state machine that punches in fast on combat start (600ms) and
exhales out slow after combat (3000ms). A 5s linger timer keeps battle
music playing across back-to-back fights during grinding, so the mood
doesn't whiplash on every encounter gap.

- Battle track selection keys off (currentZone, currentBattle) tuple
- Zone change mid-battle switches to new zone's battle track
- Fixes pre-existing autoplay bug: Howler.ctx.resume() on first pointer/
  key/touch gesture after auto-enable, so ambient actually plays without
  requiring the player to click the sound toggle
- Reorder App.tsx providers (BattleProvider now wraps SoundProvider) so
  SoundContext can read combat state

19 vitest cases cover: toggle/persistence, zone ambient, battle entry,
zone-2 battle track, zone change during combat, linger cancel-on-reentry,
linger-fires-after-timeout, mute mid-battle, autoplay unlock.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Two fixes from deep review:

1. battleMode was initialized to false even when currentBattle was
   already non-null on mount, causing a one-tick flash of ambient music
   when loading a page mid-combat. Initialize from currentBattle so
   the first playback effect run picks the correct track.

2. Unknown zones (e.g. when zone 3 ships before its music does) now
   fall back to zone 1's track with a one-time console.warn per
   (mode, zone) pair, instead of going silently off.

Adds two tests: battle track resumes on mute/unmute mid-combat, and
zone-99 combat falls back to the dark-cave battle track.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>