feat: add Groups tab with hierarchy management and member/role assignment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/pages/Admin/RbacPage.tsx

@@ -5,8 +5,6 @@ import styles from './UserManagement.module.css';
 import GroupsTab from './GroupsTab';
 import RolesTab from './RolesTab';
 
-// Placeholder component for Users tab (task 17)
-const UsersTab = () => <div>Users tab — coming soon</div>;
 
 export default function RbacPage() {
   const { data: stats } = useRbacStats();