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-compose.yml

@@ -9,6 +9,7 @@ services:
       - "8080:80"          # route this port through your reverse proxy
     environment:
       - NAVIDROME_URL=${NAVIDROME_URL}   # pre-filled on the setup screen
+      - BACKEND_URL=${BACKEND_URL}       # pre-filled on the setup screen
 
   # ── Backend (yt-dlp download server) ─────────────────────────────────────────
   novasonic-backend: