feat: add admin tab navigation between subpages

Add AdminLayout wrapper with Tabs component for navigating between
admin sections: User Management, Audit Log, OIDC, Database, OpenSearch.

Nest all /admin/* routes under AdminLayout using React Router's
Outlet pattern so the tab bar persists across admin page navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/AdminLayout.tsx

@@ -0,0 +1,26 @@
+import { Outlet, useNavigate, useLocation } from 'react-router';
+import { Tabs } from '@cameleer/design-system';
+
+const ADMIN_TABS = [
+  { label: 'User Management', value: '/admin/rbac' },
+  { label: 'Audit Log', value: '/admin/audit' },
+  { label: 'OIDC', value: '/admin/oidc' },
+  { label: 'Database', value: '/admin/database' },
+  { label: 'OpenSearch', value: '/admin/opensearch' },
+];
+
+export default function AdminLayout() {
+  const navigate = useNavigate();
+  const location = useLocation();
+
+  return (
+    <div>
+      <Tabs
+        tabs={ADMIN_TABS}
+        active={location.pathname}
+        onChange={(path) => navigate(path)}
+      />
+      <Outlet />
+    </div>
+  );
+}