[PM-5997] Resolve Opportunities QA and follow-up tickets

[jmgasper]

Summary

This batch resolves the current Opportunities QA failures and the newly added challenge-detail follow-ups while preserving tickets already in Done/QA Ready.

• correct Engagement status precedence, terms-modal lifecycle, Marathon copy/process values, and Standard copilot prize sorting
• align Marathon Match navigation, tables, score polling, history, artifact downloads, winner ratings, and modal layout
• align challenge header statuses/actions, prize artwork, rail content/icons, learning links, spacing, ratings, and scroll behavior with the authored design
• update registration/submission tables and counters immediately, and prevent unregistration after a submission
• keep an active upload mounted by locking every competing navigation/action until it completes or is explicitly cancelled
• make forum post deletion admin-only, add Copilot badges, and retain chronological comments/replies

Ticket coverage

• QA failures: PM-6075, PM-6141, PM-6143, PM-6164, PM-6166, PM-6174
• Marathon Match: PM-6181, PM-6184, PM-6186, PM-6188, PM-6189, PM-6202, PM-6206, PM-6207, PM-6209
• Forum UI: PM-6183, PM-6185
• Challenge details/design: PM-6191, PM-6193, PM-6194, PM-6195, PM-6196, PM-6197, PM-6198, PM-6199, PM-6200, PM-6201, PM-6203, PM-6204, PM-6205, PM-6208

PM-6187 and PM-6190 are intentionally excluded pending answers to the product/design clarification comments already posted in Jira.

API dependencies

• Forum authorization, email-bus configuration, Copilot projection, and ordering: [PM-5997] Resolve forum QA follow-ups forums-api-v6#12
• PM-6095 forum-style hosted attachment upload is separate: fix(PM-6095): use forum-style hosted attachment uploads support-api-v6#14

Validation

• 13 focused suites, 183 tests passed
• full yarn lint passed
• full production yarn run build passed
• independent code review completed; both upload-unmount races and overlapping artifact-download handling were corrected before push

The production build retains the repository's existing source-map and CSS-order warnings.