refactor: marketplace release polish

Sprint 28, scoped to admin/* (user decision — full marketplace audit
declined in favor of a bounded pass over the 8 admin features from
Sprints 20-27).

- a11y: aria-label added to every bare <select> not already inside a
  <label> across categories/products/orders/transactions/users/monitoring
- loading states: app-skeleton rows/cards added to list pages that
  previously rendered blank during the initial fetch (categories, orders,
  transactions, users, monitoring's event feed, analytics summary cards)
- admin-dashboard-card's custom shimmer CSS replaced with the shared
  SkeletonComponent (same visual result, one less duplicated animation)
- bundle-size budget warning (~198kB over) confirmed pre-existing —
  present at Sprint 20's first build before any admin/* code existed,
  and new admin pages are all lazy-loaded — documented as out of scope
  for this pass rather than chased

docs/ADMIN.md + docs/SPRINT-PLAN.md updated with the scope decision and
what was explicitly not done (Lighthouse, animations, SEO/sitemap,
storefront/editor a11y).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/SPRINT-PLAN.md

@@ -69,9 +69,11 @@ Notify user: **from Sprint 20 (Categories) once product↔category link + admin
 - [x] `docs/ADMIN.md` (new Sprint 27 section), `docs/BACKEND.md` item 16 added
 - Commit: `feat(admin): analytics dashboard`
 
-## Sprint 28 — Marketplace Polish
-- [ ] Lighthouse + a11y sweep, animations, skeletons/empty/error states, responsive fixes, SEO/meta/social preview/robots/sitemap
+## Sprint 28 — Marketplace Polish ✅ done (scoped to admin/* per user decision)
+- [x] A11y sweep (aria-label on unlabeled selects), loading skeletons on list pages that previously showed blank tables, bundle-size warning confirmed pre-existing/out-of-scope
+- [x] `docs/ADMIN.md` new Sprint 28 section documenting scope decision + what was explicitly not done
 - Commit: `refactor: marketplace release polish`
+- Not done (full marketplace audit was declined): Lighthouse run, animations, SEO/meta/sitemap, storefront/editor a11y.
 
 ## Sprint 29 — Release Candidate
 - [ ] Dead code cleanup, remove TODOs/console logs/debug code, optimize imports/bundle/images/CSS, translation validation, build/type/lint validation, doc validation, arch review, CHANGELOG + release notes