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>

src/app/features/admin/analytics/pages/admin-analytics-page.component.scss

@@ -11,3 +11,4 @@
 .chart .bar { flex: 1; background: var(--color-primary, #2f8f5b); border-radius: 3px 3px 0 0; min-height: 2px; }
 .pending-section p { display: flex; align-items: center; gap: 8px; margin: 0; color: var(--text-secondary, #6b7280); }
 @media (max-width: 900px) { .summary-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); } }
+.skeleton-rows { display: grid; gap: 8px; }