Replicate the desktop sign up experience on mobile

### Description

As per @JoycePlaysFootball’s feedback, we’d like to replicate the sign up desktop experience in the short term. In the long term, we can rethink it.

Current mobile page:

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

No files or tests are named. Start by opening the current mobile sign-up page and comparing it with the desktop sign-up experience; done means the mobile experience matches the desktop version shown by the issue’s goal and reference image.

Written by the indexing model from the issue text.