feat: add a recently-viewed commitments rail to the detail page

[mikewheeleer]

Summary

Context: as noted on #924's PR, this repo has a large amount of previously-deleted UI infrastructure from a prior incident (a single commit deleted ~2,500 files / 170K lines). There's no "My Commitments" list page currently -- it was among the deleted files and hasn't been rebuilt -- so per maintainer guidance I'm wiring this feature into the commitment detail page instead, the one real, existing page in the commitments domain.

What this adds

• Generalizes useRecentlyViewed to accept an optional storageKey parameter (defaulting to the existing 'marketplace-recently-viewed' key, so its current behavior and all existing tests are unchanged) instead of duplicating the whole hook for a second domain. Adds RECENTLY_VIEWED_COMMITMENTS_KEY as the commitments-domain key.
• Adds RecentlyViewedCommitmentsRail, a small presentational component (pure props in, renders nothing when the entry list is empty).
• The commitment detail page (src/app/commitments/[id]/page.tsx) now records each view via addView and renders a rail of other recently-viewed commitments (current one excluded) in the sidebar, each linking to its own detail page.

Testing

• useRecentlyViewed.test.ts -- added a test confirming a custom storageKey tracks independently of the default marketplace key (reads/writes don't cross-contaminate).
• RecentlyViewedCommitmentsRail.test.tsx -- 3 tests (renders nothing when empty, renders a link per entry with correct href/labels, accessible nav label).
• All existing tests for useRecentlyViewed still pass unchanged.

Note on verification: this page doesn't currently build end-to-end (pre-existing, unrelated missing modules -- @/lib/clientEnv, @/components/ErrorBoundary, @/components/dashboard/CommitmentHealthMetrics, etc. -- part of the same deletion incident, same as #924). Confirmed via git stash A/B comparison that this diff adds zero new build errors (identical Module not found list with and without this change) and zero new test failures.

Closes #968

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