Add NAVIDROME_URL autofill, remove Navidrome scan trigger

- Frontend reads window.NS_SERVER_URL from config.js (injected at container start via docker-entrypoint.sh) and pre-fills the setup screen
- Removed triggerNaviScan from frontend and backend (Navidrome auto-scans on file change)
- Removed NAVIDROME_USER/PASS from backend and compose

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

index.html

@@ -11,6 +11,8 @@
 <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 src="app.js" defer></script>
 </head>
 <body>
 
@@ -454,7 +456,6 @@
 <audio id="audio-element"></audio>
 <audio id="preview-audio"></audio>
 
-<script src="app.js" defer></script>
 <!-- CONTEXT MENU -->
 <div class="ctx-menu" id="ctx-menu" style="display:none"></div>