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>

server/src/Hokm.Server/Profiles/ProfileModels.cs

@@ -41,7 +41,7 @@ public class ProfileDto
     public long? PlanUntil { get; set; }
     public int Level { get; set; } = 1;
     public int Xp { get; set; }
-    public int Coins { get; set; } = 1000;
+    public int Coins { get; set; } = 2000;
     public int Rating { get; set; } = 1000;
     public StatsDto Stats { get; set; } = new();
     public List<string> OwnedAvatars { get; set; } = new() { "a-fox", "a-lion" };