feat: add UserManagement container and UsersTab

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

src/pages/Admin/UserManagement/UserManagement.tsx

@@ -0,0 +1,27 @@
+import { useState } from 'react'
+import { AdminLayout } from '../Admin'
+import { Tabs } from '../../../design-system/composites/Tabs/Tabs'
+import { UsersTab } from './UsersTab'
+import { GroupsTab } from './GroupsTab'
+import { RolesTab } from './RolesTab'
+
+const TABS = [
+  { label: 'Users', value: 'users' },
+  { label: 'Groups', value: 'groups' },
+  { label: 'Roles', value: 'roles' },
+]
+
+export function UserManagement() {
+  const [tab, setTab] = useState('users')
+
+  return (
+    <AdminLayout title="User Management">
+      <Tabs tabs={TABS} active={tab} onChange={setTab} />
+      <div style={{ marginTop: 16 }}>
+        {tab === 'users' && <UsersTab />}
+        {tab === 'groups' && <GroupsTab />}
+        {tab === 'roles' && <RolesTab />}
+      </div>
+    </AdminLayout>
+  )
+}