[Startup] "Initializing" spinner gets stuck on startup

[gino-m]

This occurs when the app is opened in the background (e.g., via Android Studio) while the phone is locked, or after completing the Google Play Services install flow and returning to the app.

The root cause is that the sign in event occurs while the MainActivity isn't running. MainViewModel emits the navigationRequests, but MainActivity is not there to receive it. To resolve this:

• The signInState in AuthenticationManager should be a StateFlow. We have one now, but we convert to a Flow because we call filterNotNull().
• Either Navigation events should only be triggered from within the Fragment lifecycle.