Connect a Firebase Realtime Database for live cross-device sync

1. 1Go to console.firebase.google.com → Add project
2. 2In the project: Build → Realtime Database → Create database → choose a region → Start in test mode
3. 3Project Settings → Your apps → Add a web app → copy the firebaseConfig object and paste it below
4. 4The copied config won't have databaseURL — you must add it manually. Format: https://<project-id>-default-rtdb.firebaseio.com
   Example: if projectId is my-app-abc, the URL is https://my-app-abc-default-rtdb.firebaseio.com