One-time setup (do once, then reuse on every counter phone):
1. Go to console.firebase.google.com β create a project.
2. Build β Realtime Database β Create β start in test mode.
3. Project settings β Your apps β Web app β copy the firebaseConfig.
4. Paste it below and tap Connect. Repeat step 4 on each phone.
Opens in Excel. For Google Sheets: open Sheets β File β Import β Upload the downloaded file.