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/AdminSettingsPage.tsx

@@ -0,0 +1,70 @@
+import { useEffect, useState } from 'react';
+import { Database } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import { Spinner } from '@/components/ui/spinner';
+import { notify } from '@/lib/notify';
+import { api } from '../../api/client';
+import { useAdminGuard } from '../../layouts/AdminLayout';
+import type { AdminSettings } from '../../api/types';
+
+export default function AdminSettingsPage() {
+  const { ready } = useAdminGuard();
+  const [settings, setSettings] = useState<AdminSettings | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [backing, setBacking] = useState(false);
+
+  useEffect(() => {
+    if (!ready) return;
+    api.adminSettings()
+      .then(setSettings)
+      .finally(() => setLoading(false));
+  }, [ready]);
+
+  const handleBackup = async () => {
+    setBacking(true);
+    try {
+      const r = await api.adminBackup();
+      notify.success(r.message);
+      window.location.href = r.download;
+    } catch (e: unknown) {
+      notify.error(e instanceof Error ? e.message : '备份失败');
+    } finally {
+      setBacking(false);
+    }
+  };
+
+  if (!ready || loading) {
+    return <div className="flex justify-center py-16"><Spinner size="lg" /></div>;
+  }
+  if (!settings) return null;
+
+  return (
+    <div className="admin-page">
+      <div className="admin-page-head">
+        <h1>系统设置</h1>
+        <p>数据目录、敏感词配置与数据库备份</p>
+      </div>
+
+      <div className="admin-card">
+        <div className="admin-card-head">运行信息</div>
+        <dl className="admin-dl">
+          <dt>数据目录</dt><dd><code>{settings.data_dir}</code></dd>
+          <dt>数据库路径</dt><dd><code>{settings.db_path}</code></dd>
+          <dt>敏感词配置</dt><dd><code>{settings.filter_path}</code></dd>
+          <dt>监听端口</dt><dd>{settings.port}</dd>
+        </dl>
+      </div>
+
+      <div className="admin-card">
+        <div className="admin-card-head">数据备份</div>
+        <p className="admin-card-desc">
+          导出当前 SQLite 数据库副本，文件名格式为 <code>jiang13_backup_YYYYMMDD_HHMMSS.db</code>
+        </p>
+        <Button onClick={handleBackup} loading={backing}>
+          <Database size={16} />
+          立即备份并下载
+        </Button>
+      </div>
+    </div>
+  );
+}