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/components/GlobalMusicPlayer.tsx

@@ -1,7 +1,7 @@
 import { useEffect } from 'react';
 import { useLocation } from 'react-router-dom';
 import { useAmbientMusic } from '../context/AmbientMusicContext';
-import { resolvePageAmbientIntensity } from '../audio/ambientMusic';
+import { ambientMusicPlayer, FEATURED_MUSIC_TITLE, resolvePageAmbientIntensity } from '../audio/ambientMusic';
 import './GlobalMusicPlayer.css';
 
 export default function GlobalMusicPlayer() {
@@ -14,13 +14,19 @@ export default function GlobalMusicPlayer() {
     setPageIntensity(routeIntensity);
   }, [routeIntensity, setPageIntensity]);
 
+  useEffect(() => {
+    const path = location.pathname.split('?')[0].replace(/\/$/, '') || '/';
+    if (path === '/dashboard') void ambientMusicPlayer.tryPlay();
+  }, [location.pathname]);
+
   return (
     <div
       className={`global-music-player${isDim ? ' global-music-player--ambient-dim' : ''}`}
       data-sfx="off"
       data-ambient-intensity={pageIntensity.toFixed(2)}
       role="region"
-      aria-label="Background music controls"
+      aria-label={`Background music — ${FEATURED_MUSIC_TITLE}`}
+      title={FEATURED_MUSIC_TITLE}
     >
       <button
         type="button"