Add border to the user id input field on the login screen

Design feedback:

it was hard to distinguish and understand at first that this is actually an input field. ...
Introducing a classic input field UI element would solve this issue.

I think all that's necessary here is to add the text field style with the border so it looks more obviously like an input field. https://developer.apple.com/documentation/swiftui/textfieldstyle/roundedborder