feat(ST-nma6ylbl): requireAdmin() guard + /admin layout-shell + tests

- lib/auth-guard.ts: requireAdmin() toegevoegd — redirect /dashboard bij !userId of !isAdmin
- app/(app)/admin/layout.tsx: admin-sidebar met links naar /admin/users, /admin/jobs, /admin/products
- app/(app)/admin/page.tsx: redirect-stub naar /admin/users
- __tests__/lib/auth-guard.test.ts: 3 tests voor requireAdmin() (geen userId, isAdmin=false, isAdmin=true)

app/(app)/admin/page.tsx

@@ -0,0 +1,5 @@
+import { redirect } from 'next/navigation'
+
+export default function AdminPage() {
+  redirect('/admin/users')
+}