feat: 新增管理员用户管理功能与账号封禁体系

此提交完成了全站管理员后台核心功能开发：
1.  新增AdminUserService与管理接口，支持用户列表查询、角色变更与封禁解封
2.  重构认证链路，统一账号封禁错误码与处理逻辑
3.  新增全局强制下线机制，账号被封禁时自动弹出告知弹窗并清理登录态
4.  重构管理员后台路由与布局，统一权限校验逻辑
5.  优化通知轮询逻辑，页面切回前台时立即刷新未读计数
6.  合并原公告、外观管理页到统一后台面板

frontend/app/admin/users/page.tsx

@@ -0,0 +1,49 @@
+import type { Metadata } from "next";
+import { cookies } from "next/headers";
+import { authCookieHeader } from "@/lib/cookies";
+import { fetchAdminUsers, type AdminUsersResponse } from "@/lib/api";
+import { getMeCached } from "@/lib/serverData";
+import UsersAdmin, { type UserFilter } from "./UsersAdmin";
+
+export const metadata: Metadata = {
+  title: "用户管理",
+};
+
+interface UsersPageProps {
+  searchParams: Promise<{ q?: string; filter?: string; page?: string }>;
+}
+
+// 筛选项映射为后端查询参数：管理员走 role，封禁走 status，其余不过滤
+function filterToQuery(filter: UserFilter): { role?: string; status?: string } {
+  if (filter === "admin") return { role: "admin" };
+  if (filter === "banned") return { status: "banned" };
+  return {};
+}
+
+// 管理员鉴权由 app/admin/layout.tsx 统一承担；列表首屏 SSR 直出，F5 无骨架闪动
+export default async function AdminUsersPage({ searchParams }: UsersPageProps) {
+  const sp = await searchParams;
+  const q = sp.q?.trim() || "";
+  const filter: UserFilter = sp.filter === "admin" || sp.filter === "banned" ? sp.filter : "all";
+  const page = Math.max(1, parseInt(sp.page || "1", 10) || 1);
+
+  const cookie = authCookieHeader(await cookies());
+  const me = await getMeCached(cookie || undefined);
+
+  let initial: AdminUsersResponse | null = null;
+  let initError = "";
+  try {
+    initial = await fetchAdminUsers({ page, q, ...filterToQuery(filter) }, cookie || undefined);
+  } catch (e) {
+    initError = e instanceof Error ? e.message : "获取用户列表失败";
+  }
+
+  return (
+    <UsersAdmin
+      initial={initial}
+      initError={initError}
+      initialQuery={{ q, filter, page }}
+      currentUserId={me.user?.id ?? 0}
+    />
+  );
+}