PR 7c - feat(marketplace): preview attempt frontend

[LWS49]

Summary

Phase 3 (frontend) of the marketplace preview attempt, stacked on #8511 (PR7b, the backend serving stack). This adds the frontend for previewing a published marketplace assessment as a real, interactive attempt: an entrance from the listing preview page and from each marketplace index row, the interactive attempt page itself, and a banner marking it as a throwaway preview. The attempt page reuses the real submission editor (SubmissionEditIndex) wholesale, since PR7b deliberately made the preview serving payload shape-compatible with the real submission edit.json. The only new machinery is a small context seam so the reused submission API routes to the shallow /marketplace/attempt/... endpoints, plus the banner and the two entrances. No backend changes.

Design decisions

• Reuse SubmissionEditIndex wholesale instead of forking it - PR7b's payload is shape-compatible on purpose, and forking would duplicate the ~30-component submission editor and guarantee the two experiences drift.
• Route the reused submission/scribing APIs to the preview endpoints via a module-level getActivePreview() singleton that swaps the URL prefix to /courses/:c/marketplace/attempt. The swap also triggers whenever the URL carries no assessment id, so a poller firing after the preview page unmounts can never emit a /assessments/null/... request (the fallback the repo's preview note calls for).
• Set the preview-context singleton synchronously in the route wrapper's render body, not in a useEffect - the reused class component dispatches its fetch in componentDidMount, which runs before a parent component's effects, so an effect would set the flag too late for the first request.
• Keep the React context and banner in the submission bundle (not marketplace) so the dependency direction stays marketplace -> submission (the marketplace route already imports the submission editor).
• Provide two entrances (the listing preview page and each marketplace index row), sharing one useStartPreviewAttempt hook for the create -> navigate -> 409-notify flow.

Regression prevention

• Covers: the getActivePreview singleton lifecycle; submission, scribing, and create-attempt URL routing including the assessments/null fallback (mutation-verified); the preview banner render plus its Reset and Exit actions; the route wrapper's synchronous singleton set, unmount cleanup, and course-context forwarding (mutation-verified); both entrances (create -> navigate on success, notification and no navigation on 409); and the index row play button's position between the preview and duplicate buttons and its callback.
• The banner also mounts on the shared real-submission edit page and self-hides for non-preview submissions; it reads the course outlet context defensively so it can never throw there, and the route wrapper forwards that context so the reused editor keeps working under the preview route.
• Backward compatible: the submission API's preview branch is inert on real submission URLs (which always carry an assessment id), and the new onAttempt table prop is optional, so existing submission and marketplace behaviour is unchanged.