Can't mix author/publish roles when adding multiple people in user admin

When adding more than one user via user-admin tool, if you give the first person `publish`, author is disabled throughout for any subsequent users added via the modal.

Reproduce the issue in the user-admin tool by adding multiple people through the modal, selecting publish for the first person, and observing the role controls for later users. Trace the modal's role-selection state and verify that author and publish can be mixed independently for each added user, with regression coverage if the existing test structure supports it.

Written by the indexing model from the issue text.