Fix Firebase Google Sign-In redirect loop in production

Problem: Firebase Google Authentication works in embedded app viewer but fails in published version. After OAuth popup, user is redirected back to login form. Stuck in infinite loop.

Fix 1 - google_sign_in 7.2.0 breaking changes:
google_sign_in 7.2.0 is a major breaking update. Code from previous versions will fail silently. Use the complete 7.2.0 example from official docs, not old tutorials. Don't mix web and android/ios login paths.

Fix 2 - Add custom domain to authorized domains:

Firebase Console → Authentication → Settings → Authorized domains
Add: your-custom-domain.com

Fix 3 - Check OAuth redirect URI configuration:

Google Cloud Console → APIs & Services → Credentials
Authorized redirect URIs:
- https://your-app.firebaseapp.com/__/auth/handler
- https://your-custom-domain.com/__/auth/handler

Debug steps:

1. Check browser console for CORS/redirect errors
2. Verify Firebase config matches between environments
3. Clear cache and test in incognito
4. Ensure apiKey, authDomain are correct

Source: https://www.reddit.com/r/Firebase/comments/1r52p89/firebase_authentication_issue/