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/core/auth/api.ts

@@ -3,6 +3,20 @@ import { getBackendBaseURL } from "@/core/config";
 
 import type { LoginByUsernameResponse } from "./index";
 
+export interface UserListItem {
+  id: string;
+  email: string;
+  system_role: "admin" | "user";
+  skill_count: number;
+  agent_count: number;
+}
+
+export async function listAllUsers(): Promise<UserListItem[]> {
+  const res = await apiFetch(`${getBackendBaseURL()}/api/v1/auth/users`);
+  if (!res.ok) throw new Error(`HTTP ${res.status}`);
+  return res.json() as Promise<UserListItem[]>;
+}
+
 export async function loginByUsername(
   username: string,
 ): Promise<LoginByUsernameResponse> {

frontend-web/src/core/auth/hooks.ts

@@ -0,0 +1,11 @@
+import { useQuery } from "@tanstack/react-query";
+
+import { listAllUsers } from "./api";
+
+export function useAllUsers(enabled: boolean) {
+  return useQuery({
+    queryKey: ["admin", "users"],
+    queryFn: listAllUsers,
+    enabled,
+  });
+}