Add Auth\UserName::format(), which prefers a user's first + last name, then
their nickname, avoiding display_name (which can be the login/username).
Route the instructor name through it in both the front-end offerings response
(instructor_name) and the back-end group-class summary and details views.
Tests: composer test (513), composer lint, composer cs all pass.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
