refactor: convert the requestCert writer to a React Query mutation (#1995)

The shared `requestCert` POST thunk is used by three tabs — CertificateStatusAlert
(outline), CourseCelebration (course-exit), and CertificateStatus (progress).
Convert all three to a `useRequestCert` mutation (fire-and-forget POST to
generate_user_cert) and delete the thunk. Prerequisite below the outline-tab
conversion so outline builds on an already-converted CertificateStatusAlert.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>