music: re-enable background loop (default = playful) + profile on/off; coins 2000

- sound.ts: restored startMusic (was a no-op stub) playing the selected track
  through musicGain (in-game mute still applies); default track switched to
  "playful" (per user). Music auto-starts on init when enabled.
- Profile → Audio: re-added a music on/off toggle (so players can disable it
  outside the game too); SFX toggle unchanged.
- Economy tune: starting coins 1000 → 2000 (mock defaultProfile + server
  ProfileDto) so new players start a bit richer.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/online/mock-service.ts

@@ -128,7 +128,7 @@ function defaultProfile(session: AuthSession): UserProfile {
     plan: "free",
     level: 1,
     xp: 0,
-    coins: 1000,
+    coins: 2000,
     rating: 1000,
     stats: {
       games: 0,