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>

nginx.conf

@@ -24,6 +24,11 @@ server {
         add_header Service-Worker-Allowed "/";
     }
 
+    # Runtime config – never cache so NAVIDROME_URL changes are picked up
+    location = /config.js {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+    }
+
     # SPA fallback
     location / {
         try_files $uri $uri/ /index.html;