Signup: Emails should be lowercased before sending to the backend

### Expected

If a user enters an email using some capital letters, it should be converted to lowercase on form submission.

### Observed

The app sends a mixed-case email to the backend for registration.

Further context in pbMoDN-2oI-p2

Start at the signup form's submission path and trace the registration request to the backend. Ensure the submitted email is lowercase, then verify the request behavior with the existing signup or registration tests; done means mixed-case input reaches the backend in lowercase.

Written by the indexing model from the issue text.