This reverts the change from 90c067997b so that the username is shown in situations where we don't have a valid session but could almost certainly get one. Instead, to achieve what that commit was trying to do, we'll instead remove the username from UserDefaults if auth fails with an invalid username or password error. I think this will more closely match what users expect in this UI.
I've added a comment in the UI explaining why it is the way it is. It might also be worth considering renaming AuthenticationState or its cases to better reflect that it's probably more about the (short-lived) session state than whether the user has signed in before and has stored their credentials.
