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/page.tsx

@@ -0,0 +1,323 @@
+import type { Metadata } from "next";
+import Link from "next/link";
+import { cookies } from "next/headers";
+import {
+  Users,
+  PenLine,
+  MessageCircle,
+  Activity,
+  Megaphone,
+  Palette,
+  ChevronRight,
+  UserPlus,
+  LayoutGrid,
+} from "lucide-react";
+import { fetchOverview, type OverviewResponse } from "@/lib/api";
+import { authCookieHeader } from "@/lib/cookies";
+import { getMeCached } from "@/lib/serverData";
+import Avatar from "@/components/Avatar";
+import { formatRelative } from "@/lib/format";
+
+export const metadata: Metadata = {
+  title: "管理仪表盘",
+};
+
+// 精确千分位（管理员视角需要真实数值，不用首页的紧凑 K/M）
+const fmt = (n: number) => n.toLocaleString("zh-CN");
+
+function greeting(hour: number) {
+  if (hour < 6) return "凌晨好";
+  if (hour < 12) return "上午好";
+  if (hour < 14) return "中午好";
+  if (hour < 18) return "下午好";
+  return "晚上好";
+}
+
+// 统计 Bento 卡：总量 + 今日增量，语义色与全站令牌一致
+function StatCard({
+  label,
+  value,
+  foot,
+  icon,
+  color,
+  soft,
+}: {
+  label: string;
+  value: string;
+  foot: string;
+  icon: React.ReactNode;
+  color: string;
+  soft: string;
+}) {
+  return (
+    <div className="panel p-4 sm:p-5">
+      <div className="flex items-start justify-between gap-2">
+        <p className="text-[12.5px] font-semibold" style={{ color: "var(--ink-3)" }}>
+          {label}
+        </p>
+        <span
+          className="w-8 h-8 rounded-lg flex items-center justify-center shrink-0"
+          style={{ background: soft, color }}
+          aria-hidden="true"
+        >
+          {icon}
+        </span>
+      </div>
+      <p
+        className="mt-3 text-[26px] sm:text-[30px] font-extrabold tabular-nums leading-none tracking-tight"
+        style={{ color: "var(--ink)" }}
+      >
+        {value}
+      </p>
+      <p className="mt-2 text-[11.5px] sm:text-[12px] font-semibold" style={{ color: "var(--ink-3)" }}>
+        {foot}
+      </p>
+    </div>
+  );
+}
+
+// 快捷管理入口：整卡可点（stretch-link），图标 + 标题 + 描述 + 箭头
+function QuickLink({
+  href,
+  icon,
+  title,
+  desc,
+  soft,
+  color,
+}: {
+  href: string;
+  icon: React.ReactNode;
+  title: string;
+  desc: string;
+  soft: string;
+  color: string;
+}) {
+  return (
+    <Link href={href} className="panel hover-lift relative block p-5 group">
+      <div className="flex items-start gap-3.5">
+        <span
+          className="w-10 h-10 rounded-xl flex items-center justify-center shrink-0"
+          style={{ background: soft, color }}
+          aria-hidden="true"
+        >
+          {icon}
+        </span>
+        <div className="min-w-0 flex-1">
+          <p className="text-[15px] font-extrabold" style={{ color: "var(--ink)" }}>
+            {title}
+          </p>
+          <p className="meta mt-1 text-[12.5px] leading-relaxed">{desc}</p>
+        </div>
+        <ChevronRight
+          size={17}
+          className="shrink-0 mt-1 transition-transform group-hover:translate-x-0.5"
+          style={{ color: "var(--ink-3)" }}
+        />
+      </div>
+    </Link>
+  );
+}
+
+function PanelHead({ title, extra }: { title: string; extra?: string }) {
+  return (
+    <div
+      className="px-4 sm:px-5 py-3.5 flex items-center justify-between gap-2"
+      style={{ borderBottom: "1px solid var(--line)" }}
+    >
+      <span className="font-bold text-[15px]" style={{ color: "var(--ink)" }}>
+        {title}
+      </span>
+      {extra && (
+        <span className="meta text-[12px] tabular-nums" style={{ color: "var(--ink-3)" }}>
+          {extra}
+        </span>
+      )}
+    </div>
+  );
+}
+
+export default async function AdminDashboardPage() {
+  const cookie = authCookieHeader(await cookies());
+  // 与 layout 的 /me 请求共享 React cache；概览失败降级，不阻塞仪表盘骨架
+  const [me, overview] = await Promise.all([
+    getMeCached(cookie || undefined),
+    fetchOverview(cookie).catch(() => null),
+  ]);
+
+  const now = new Date();
+  const dateLabel = new Intl.DateTimeFormat("zh-CN", {
+    year: "numeric",
+    month: "long",
+    day: "numeric",
+    weekday: "long",
+  }).format(now);
+
+  const stats = overview?.stats;
+  const boards = overview?.boards ?? [];
+  const newUsers = overview?.new_users ?? [];
+  const topBoards = [...boards].sort((a, b) => b.post_count - a.post_count).slice(0, 6);
+  const maxBoardCount = Math.max(1, ...topBoards.map((b) => b.post_count));
+
+  return (
+    <div>
+      {/* 页头 */}
+      <div className="mb-5">
+        <h1 className="text-[22px] sm:text-[26px] font-extrabold tracking-tight" style={{ color: "var(--ink)" }}>
+          {greeting(now.getHours())}，{me.user?.nickname}
+        </h1>
+        <p className="meta mt-1.5 text-[12.5px]">{dateLabel} · 这里是站点运营全貌</p>
+      </div>
+
+      {/* 统计 Bento */}
+      <div className="grid grid-cols-2 lg:grid-cols-4 gap-3 sm:gap-4">
+        <StatCard
+          label="注册用户"
+          value={stats ? fmt(stats.users) : "—"}
+          foot={stats ? `今日新增 +${fmt(stats.today_users)}` : "统计暂不可用"}
+          icon={<Users size={16} />}
+          color="var(--accent)"
+          soft="var(--accent-soft)"
+        />
+        <StatCard
+          label="话题总数"
+          value={stats ? fmt(stats.posts) : "—"}
+          foot={stats ? `今日新发 +${fmt(stats.today_posts)}` : "统计暂不可用"}
+          icon={<PenLine size={16} />}
+          color="var(--clay)"
+          soft="var(--clay-soft)"
+        />
+        <StatCard
+          label="回复总数"
+          value={stats ? fmt(stats.comments) : "—"}
+          foot={stats ? `今日新增 +${fmt(stats.today_comments)}` : "统计暂不可用"}
+          icon={<MessageCircle size={16} />}
+          color="var(--ok)"
+          soft="var(--ok-soft)"
+        />
+        <StatCard
+          label="在线用户"
+          value={stats ? fmt(stats.online) : "—"}
+          foot={stats ? `近 5 分钟活跃 · 峰值 ${fmt(stats.peak_online)}` : "统计暂不可用"}
+          icon={<Activity size={16} />}
+          color="var(--gold)"
+          soft="var(--gold-soft)"
+        />
+      </div>
+
+      {/* 快捷入口 */}
+      <h2 className="sr-only">快捷管理</h2>
+      <div className="grid sm:grid-cols-2 xl:grid-cols-3 gap-3 sm:gap-4 mt-4">
+        <QuickLink
+          href="/admin/users"
+          icon={<Users size={18} />}
+          title="用户管理"
+          desc="查看全站成员，授予管理员权限或封禁违规账号"
+          soft="var(--accent-soft)"
+          color="var(--accent)"
+        />
+        <QuickLink
+          href="/admin/announcements"
+          icon={<Megaphone size={18} />}
+          title="公告管理"
+          desc="发布、编辑与下线站点公告，支持标签颜色与草稿状态"
+          soft="var(--clay-soft)"
+          color="var(--clay)"
+        />
+        <QuickLink
+          href="/admin/appearance"
+          icon={<Palette size={18} />}
+          title="外观设置"
+          desc="配置全站主题强调色，实时预览后保存并对所有用户生效"
+          soft="var(--gold-soft)"
+          color="var(--gold)"
+        />
+      </div>
+
+      {/* 成员与板块 */}
+      <div className="grid lg:grid-cols-2 gap-4 mt-4 items-start">
+        {/* 最新注册成员 */}
+        <div className="panel overflow-hidden">
+          <PanelHead title="最新注册成员" extra={stats ? `共 ${fmt(stats.users)} 人` : undefined} />
+          {newUsers.length === 0 ? (
+            <p className="meta text-center py-14 text-[13px]">暂无成员数据</p>
+          ) : (
+            newUsers.slice(0, 6).map((u) => (
+              <Link
+                key={u.id}
+                href={`/u/${u.id}`}
+                className="flex items-center gap-3 px-4 sm:px-5 py-3 transition-colors hover:bg-[var(--panel-2)]"
+                style={{ borderTop: "1px solid var(--line)" }}
+              >
+                <Avatar name={u.nickname || u.username} src={u.avatar || undefined} size={34} />
+                <span className="min-w-0 flex-1 leading-tight">
+                  <span className="block text-[13.5px] font-bold truncate" style={{ color: "var(--ink)" }}>
+                    {u.nickname}
+                  </span>
+                  <span className="block text-[11.5px] truncate" style={{ color: "var(--ink-3)" }}>
+                    @{u.username}
+                  </span>
+                </span>
+                <span
+                  className="shrink-0 inline-flex items-center gap-1 text-[11.5px] font-semibold"
+                  style={{ color: "var(--ink-3)" }}
+                >
+                  <UserPlus size={12} aria-hidden="true" />
+                  {formatRelative(u.created_at)}加入
+                </span>
+              </Link>
+            ))
+          )}
+        </div>
+
+        {/* 板块分布 */}
+        <div className="panel overflow-hidden">
+          <PanelHead title="板块话题分布" extra={`${boards.length} 个板块`} />
+          {topBoards.length === 0 ? (
+            <p className="meta text-center py-14 text-[13px]">暂无板块数据</p>
+          ) : (
+            topBoards.map((b) => (
+              <Link
+                key={b.id}
+                href={`/?board=${b.id}`}
+                className="flex items-center gap-3 px-4 sm:px-5 py-3 transition-colors hover:bg-[var(--panel-2)]"
+                style={{ borderTop: "1px solid var(--line)" }}
+              >
+                <span
+                  className="w-8 h-8 rounded-lg flex items-center justify-center shrink-0"
+                  style={{ background: "var(--panel-2)", color: "var(--accent)" }}
+                  aria-hidden="true"
+                >
+                  <LayoutGrid size={14} />
+                </span>
+                <span className="min-w-0 flex-1">
+                  <span className="flex items-center justify-between gap-2">
+                    <span className="text-[13.5px] font-bold truncate" style={{ color: "var(--ink)" }}>
+                      {b.name}
+                    </span>
+                    <span className="text-[12px] font-bold tabular-nums shrink-0" style={{ color: "var(--ink-2)" }}>
+                      {fmt(b.post_count)}
+                    </span>
+                  </span>
+                  {/* 相对占比迷你条（纯装饰） */}
+                  <span
+                    className="block mt-1.5 h-1 rounded-full overflow-hidden"
+                    style={{ background: "var(--tint)" }}
+                    aria-hidden="true"
+                  >
+                    <span
+                      className="block h-full rounded-full"
+                      style={{
+                        width: `${Math.max(4, Math.round((b.post_count / maxBoardCount) * 100))}%`,
+                        background: "var(--accent)",
+                      }}
+                    />
+                  </span>
+                </span>
+              </Link>
+            ))
+          )}
+        </div>
+      </div>
+    </div>
+  );
+}