FTUE: Mobile Auth Community testing

### Your use case

We ran a [community testing session](https://docs.google.com/spreadsheets/d/1EshS4x2f4k-E4woLFxmGJ0q9t_QlCyGc/edit#gid=642673389), this is the feedback:

### Are you human?

- On the "I'm not a robot" screen, the background of the iframe is white and the button isn't centred
- On the "Select all images with…" screen, the screen is bigger than my phone screen and is scrollable

### Profile personalisation

- When using SSO, you're not asked to complete your profile

### Error messages

- The "capital letter in username" error message is not helpful
- App crashes entering random chars like \/{}++$$$&&&& in the username

### Back button

- It is not clear how to get back to the previous step. Common sense is to have a „go back“ link at top left of the screen.