fix(register): include username when creating user

[shivaansh0610-LUFFY]

What this does

The registration form did not include the required username field
When creating a user, which caused the backend to reject the request.

This change adds username (derived from the email) to the payload
sent to the createUser action.

Why this is needed

plone.restapi requires a username when creating users.
Without it, registration fails with a validation error.

How to test

1. Go to the registration form
2. Submit fullname + email
3. Verify that user creation succeeds

Closes #6987

[mister-roboto]

@shivaansh0610-LUFFY you need to sign the Plone Contributor Agreement to merge this pull request.

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[mister-roboto]

@shivaansh0610-LUFFY you need to sign the Plone Contributor Agreement to merge this pull request.

Learn about the Plone Contributor Agreement: https://plone.org/foundation/contributors-agreement

If you have already signed the agreement, please allow a week for your agreement to be processed.
Once it is processed, you will receive an email invitation to join the plone GitHub organization as a Contributor.

If after a week you have not received an invitation, then please contact [email protected].

[shivaansh0610-LUFFY]

CLA has been signed and processed.
Happy to update anything else or rebase if needed.