Add admin UX improvements, constants refactor, and comprehensive tests

- Introduced constants for admin hooks: `STATS_FETCH_LIMIT`, `DEFAULT_PAGE_LIMIT`, and `STATS_REFETCH_INTERVAL` to enhance readability and maintainability.
- Updated query guards to ensure data fetching is restricted to superusers.
- Enhanced accessibility across admin components by adding `aria-hidden` attributes and improving focus-visible styles.
- Simplified `useAdminStats`, `useAdminUsers`, and `useAdminOrganizations` with shared constants.
- Added 403 Forbidden page with proper structure, styling, and tests.
- Implemented new tests for admin hooks, DashboardStats, AdminLayout, and ForbiddenPage for better coverage.

frontend/src/components/admin/StatCard.tsx

@@ -90,6 +90,7 @@ export function StatCard({
               'h-6 w-6',
               loading ? 'text-muted-foreground' : 'text-primary'
             )}
+            aria-hidden="true"
           />
         </div>
       </div>