🐛 [firebase_ui_auth] problems with sign-in screen layout

## Bug report
See https://github.com/firebase/flutterfire/issues/10431 for files etc.
Issue number 5 in firebase_ui_auth 1.1.10 issues.pdf

There are problems with responsive_page.dart.
There appears to be no way of setting the width of the email/ password text boxes on sign-in or register screens and if you supply a header, it sometimes appears and sometimes doesn't and the breakpoint parameter that can be passed in isn't documented properly. Depending on the width of the browser window, the width of the text boxes is sometimes very wide, which is wrong.

The attached version of responsive_page.dart solves the problems with the layout but it has a hard-coded max width (that probably needs adjusting) for the edit boxes. Trying to use the width values in the theme for the text box widths does nothing.

see also
https://github.com/firebase/flutterfire/issues/10431
https://github.com/firebase/FirebaseUI-Flutter/issues/39
https://github.com/firebase/FirebaseUI-Flutter/issues/38
https://github.com/firebase/FirebaseUI-Flutter/issues/37
https://github.com/firebase/flutterfire/issues/10436