Fix Firebase Auth Config Error

I have a Flutter app (currently Android-only) that is almost ready for release, but the sign-up screen throws “An internal error occurred [Configuration_Not_Found]” every time a new user registers.

The Firebase project is already created, google-services.json is in the codebase, and the API keys are in place. The only Firebase service I need right now is Authentication, with App Check switched on. I’m looking for someone who can trace the misconfiguration—whether it’s a missing SHA-1/256, an App Check setting, or a console rule—and get the authentication flow running smoothly.

Acceptance criteria
• A test build registers and logs in a new user without errors.
• Firebase Authentication and App Check dashboards reflect the activity.
• You give me a concise note of what was changed so I can reproduce the setup later.