Add featured Dark House track with default landing ambient playback

Wire featured-dark-house-2025.mp3 as the default ambient source, enable BGM for first-time visitors, and attempt autoplay on Command Deck with gesture-unlock fallback. The ~57MB MP3 stays gitignored; copy instructions are in public/audio/.gitkeep.

server/web/src/context/AmbientMusicContext.tsx

@@ -63,6 +63,10 @@ export function AmbientMusicProvider({ children }: { children: React.ReactNode }
     return ambientMusicPlayer.subscribe(setPlaying);
   }, []);
 
+  useEffect(() => {
+    if (enabled) void ambientMusicPlayer.tryPlay();
+  }, []);
+
   useEffect(() => {
     ambientMusicPlayer.setEnabled(enabled);
     ambientMusicPlayer.setVolume(volume);