feat: admin skill public upload + user management page

- Admin users can choose to install skills to public (built-in) or custom category via UI radio selector in InstallSkillDialog
- Backend: SkillStorage.ainstall_skill_from_archive accepts target_category param; non-admin 403 on public install
- Backend: AgentStore/SkillStore add count_owned(user_id); UserRepository/LocalAuthProvider add list_users()
- Backend: GET /api/v1/auth/users endpoint (admin-only) returns all users with skill_count and agent_count
- Frontend: admin-only "用户管理" nav item in sidebar; /page/workspace/admin/users route with user table
- i18n: added sidebar.users and adminUsers section in zh-CN/en-US

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

frontend-web/src/pages/UsersPage.tsx

@@ -0,0 +1,5 @@
+import { UserManagementPage } from "@/components/workspace/admin/user-management-page";
+
+export default function UsersPage() {
+  return <UserManagementPage />;
+}

frontend-web/src/pages/WorkspaceRoutes.tsx

@@ -18,6 +18,7 @@ import ScheduledTaskRunDetailPage from "./ScheduledTaskRunDetailPage";
 import ScheduledTasksPage from "./ScheduledTasksPage";
 import SkillEvolutionPage from "./SkillEvolutionPage";
 import SkillsPage from "./SkillsPage";
+import UsersPage from "./UsersPage";
 
 function WorkspaceLayout({
   children,
@@ -149,6 +150,14 @@ export default function WorkspaceRoutes({
           </WorkspaceLayout>
         }
       />
+      <Route
+        path="admin/users"
+        element={
+          <WorkspaceLayout pageSidebarCollapsed={pageSidebarCollapsed}>
+            <UsersPage />
+          </WorkspaceLayout>
+        }
+      />
     </Routes>
   );
 }