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/dashboard/components/admin-dashboard-card.component.html

@@ -6,7 +6,7 @@
 
   <div class="dashboard-card__body">
     <ng-container [ngSwitch]="status">
-      <div class="dashboard-card__skeleton" *ngSwitchCase="'loading'"></div>
+      <app-skeleton *ngSwitchCase="'loading'" shape="rect" height="24px" width="60%" />
 
       <p class="dashboard-card__muted" *ngSwitchCase="'empty'">{{ 'dashboard.stateEmpty' | translate }}</p>