Fix bottom nav full-width layout + add backend URL autofill

- Remove max-width/gap/padding from bottom nav so buttons fill full screen width
- Add NS_BACKEND_URL to config.js entrypoint and autofill backend-url field on setup screen

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

docker-entrypoint.sh

@@ -1,6 +1,7 @@
 #!/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 = "${NAVIDROME_URL:-}";
+window.NS_SERVER_URL  = "${NAVIDROME_URL:-}";
+window.NS_BACKEND_URL = "${BACKEND_URL:-}";
 EOF
 exec nginx -g "daemon off;"