allow changing firebase region after logging out

[lukaskollmer]

Description

we select a firebase environment based on the region selected during the initial onboarding. this firebase environment selection is persisted to disk, as a UserDefaults entry. this allows us to load the correct environment on subsequent launches

issue: when someone logs out of their account, we present the onboarding again, as part of which they get to select a region again

• but: if the app hasn't been killed since logging out, firebase is still configured for the region they were logged in to
• meaning: that we can't actually allow them to switch to a different region, unless they terminate and re-launch the app first
• technically, it might be possible to un-configure firebase in the iOS app, but that'd be very much a non-trivial undertaking since we'd need to somehow coordinate unloading firebase itself as well all Spezi modules that depend on it and were loaded as part of configuring firebase, and some of these modules (as well as MHC itself) currently operate under the assumption that firebase is only ever loaded into the app, but never goes away

Reproduction

• log in
• log out
• select a different region than you selected initially

Expected behavior

we need to either:

• add the ability to dynamically unload firebase, in a way that it can be re-enabled when the user goes through the onboarding, OR
• keep track of the fact that the app is in a logged-out-but-still-firebase-enabled state, and somehow communicate this to the user (ie, that they need to force-quit and relaunch the app if they wish to change eg from US to UK)

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines

[lukaskollmer]

another, related issue:

1. user downloads the app, selects a region (US), creates an account, but does not complete the onboarding
2. user now re-opens the app, and goes through the onboarding again
   • if the user re-selects the same region (US), the "we see you're already logged in; do you want to use this account again" step will work w/out issues
   • but if the user now selects a different region (eg UK), we'd still offer them to use the previously-logged-in account, even though that would not work, bc the credentials are not valid with the UK firebase deployment

ideally, we'd have some kind of mechanism (via SpeziAccount, or Firebase if this is handled there) to clear the cached credentials and as a result disable the "do you want to re-use a previously-logged-in account

[lukaskollmer]

additionally: the app currently persists the last-used firebase region when firebase is loaded (i.e. immediately after selecting a region on the 2nd onboarding step); which leads to a situation where a user not completing the onboarding and then re-launching the app will, from that point onwards, always be locked in to the initially-selected region's deployment, regardless of which region they select during a subsequent launch