[Code health] Replace firebaseui

[gino-m]

From https://firebase.google.com/docs/auth/web/firebaseui:

FirebaseUI is not compatible with the modular API. The namespaced API (specifically, the app-compat and auth-compat packages) permits the usage of FirebaseUI alongside the JavaScript SDK, but without the app size reduction and other benefits of the modular API.

The latest firebaseui also requires Angular 20. This prevents us from upgrading to Angular 21, causing transitive dependency lock-in.