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>

.env.example

@@ -1,7 +1,8 @@
 # Copy to .env and fill in your values
 
-# Pre-fills the server URL on the setup screen
+# Pre-fills the URLs on the setup screen
 NAVIDROME_URL=https://music.yourdomain.com
+BACKEND_URL=https://novasonic-api.yourdomain.com
 
 # Path to your Navidrome music library on the host
 MUSIC_PATH=/path/to/your/navidrome/music