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>

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v17';
+const CACHE = 'ns-v18';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'sw.js', 'manifest.json', 'icon.svg'];
 const SKIP_URLS = ['/rest/stream', '/rest/download', '/config.js'];