Fix MUSIC_PATH direct mount + backend URL prefill

- MUSIC_PATH is now used 1:1 inside the container (no /music remapping)
- config.js 404-safe with inline fallback defaults
- Prefill uses .trim() so empty strings don't overwrite fields

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -13,7 +13,8 @@
 <link rel="preconnect" href="https://fonts.googleapis.com">
 <link href="https://fonts.googleapis.com/css2?family=Outfit:wght@300;400;500;600;700;800&family=JetBrains+Mono:wght@300;400;500&display=swap" rel="stylesheet">
 <link rel="stylesheet" href="styles.css">
-<script src="config.js"></script>
+<script>window.NS_SERVER_URL='';window.NS_BACKEND_URL='';</script>
+<script src="config.js" onerror="0"></script>
 <script src="app.js" defer></script>
 </head>
 <body>