accounts: use lowercase emails only

We had a lot of problems with people entering their emails with uppercase letters, mostly from mobile devices and the current field provides uppercase by default for the first letter.

To address those issues, but also prevent people registering multiple accounts with the same email, we are switching to only lowercase emails support.

There will be other MRs following up

Following recommendations from https://github.com/orgs/react-hook-form/discussions/7867