[HIGH PRIORITY] Implement validation for Login and use Riverpod to update current user

[michho8]

• [x] Switch from the text fields to flutter_form_builder
• [x] Check whether the email is included in the mock UsersDB
• [x] Create error messages under the text fields - snack bar?
• [x] Update the provider for current user

[michho8]

ref.read(currentUserProvider.state).state = 'user-002'; Change login to state provider