统一 React 管理后台，修复评论换行与帖子置顶

- /admin/* 全部由 React SPA 渲染，替代旧版 HTML 后台页面
- 新增仪表盘、帖子/评论/用户管理、系统设置与 JSON API
- 帖子详情页支持管理员置顶；评论换行显示修复

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/pages/admin/AdminDashboardPage.tsx

@@ -0,0 +1,88 @@
+import { useEffect, useState } from 'react';
+import { useNavigate } from 'react-router-dom';
+import { Spinner } from '@/components/ui/spinner';
+import { Badge } from '@/components/ui/badge';
+import { api } from '../../api/client';
+import { useAdminGuard } from '../../layouts/AdminLayout';
+import type { AdminDashboard } from '../../api/types';
+
+export default function AdminDashboardPage() {
+  const nav = useNavigate();
+  const { ready } = useAdminGuard();
+  const [data, setData] = useState<AdminDashboard | null>(null);
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    if (!ready) return;
+    api.adminDashboard()
+      .then(setData)
+      .finally(() => setLoading(false));
+  }, [ready]);
+
+  if (!ready || loading) {
+    return <div className="flex justify-center py-16"><Spinner size="lg" /></div>;
+  }
+  if (!data) return null;
+
+  const stats = [
+    { label: '注册用户', value: data.users, cls: 'admin-stat-users' },
+    { label: '帖子总数', value: data.posts, cls: 'admin-stat-posts' },
+    { label: '板块数量', value: data.boards, cls: 'admin-stat-boards' },
+    { label: '评论总数', value: data.comments, cls: 'admin-stat-comments' },
+    { label: '当前在线', value: data.online, cls: 'admin-stat-online' },
+  ];
+
+  return (
+    <div className="admin-page">
+      <div className="admin-page-head">
+        <h1>仪表盘</h1>
+        <p>论坛运行概览与最新帖子</p>
+      </div>
+
+      <div className="admin-stat-grid">
+        {stats.map(s => (
+          <div key={s.label} className={`admin-stat-card ${s.cls}`}>
+            <div className="admin-stat-value">{s.value}</div>
+            <div className="admin-stat-label">{s.label}</div>
+          </div>
+        ))}
+      </div>
+
+      <div className="admin-card">
+        <div className="admin-card-head">
+          <span>最新帖子</span>
+          <button type="button" className="admin-text-link" onClick={() => nav('/admin/posts')}>
+            查看全部 →
+          </button>
+        </div>
+        <table className="admin-table">
+          <thead>
+            <tr>
+              <th>ID</th>
+              <th>标题</th>
+              <th>作者</th>
+              <th>置顶</th>
+              <th>时间</th>
+            </tr>
+          </thead>
+          <tbody>
+            {data.recent_posts.map(p => (
+              <tr key={p.id}>
+                <td>{p.id}</td>
+                <td>
+                  <button type="button" className="admin-text-link" onClick={() => nav(`/post/${p.id}`)}>
+                    {p.title}
+                  </button>
+                </td>
+                <td>{p.user?.nickname ?? '—'}</td>
+                <td>{p.pinned ? <Badge variant="orange">是</Badge> : '—'}</td>
+                <td>{new Date(p.created_at).toLocaleString('zh-CN')}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+        {data.recent_posts.length === 0 && <div className="admin-empty">暂无帖子</div>}
+      </div>
+    </div>
+  );
+}