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>

docker-entrypoint.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# Generate runtime config from env vars so the frontend knows the Navidrome URL
+cat > /usr/share/nginx/html/config.js <<EOF
+window.NS_SERVER_URL = "${NAVIDROME_URL:-}";
+EOF
+exec nginx -g "daemon off;"