(See #691)

I have chosen this Activity as one where not that much happens and I think it could be implemented quite "clean".

To start, I'd:

• Convert M2 calls to M3 (without architectural changes)
• Split into 3 files (Activity, Screen, Model, see Migrate to Material 3 (M3) #691 and for instance LoginActivity) without further changes
• Change = MutableLiveData() to by mutableStateOf() for our own fields (that don't come from an external Flow) in the model

Then when this is done we can optimize the composable and models a bit and think whether we leave the rest as it is or whether further changes would be useful (like moving the debug info generation to a separate data layer class).