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>

sw.js

@@ -1,6 +1,6 @@
 const CACHE = 'ns-v12';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'sw.js'];
-const SKIP_URLS = ['/rest/stream', '/rest/download'];
+const SKIP_URLS = ['/rest/stream', '/rest/download', '/config.js'];
 
 self.addEventListener('install', e => {
   e.waitUntil(caches.open(CACHE).then(c => c.addAll(PRECACHE)));