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/layouts/AdminLayout.tsx

@@ -0,0 +1,97 @@
+import { useEffect } from 'react';
+import { Outlet, NavLink, useNavigate } from 'react-router-dom';
+import {
+  LayoutDashboard, FolderKanban, FileText, MessageSquare, Users, Settings, ArrowLeft,
+} from 'lucide-react';
+import { Spinner } from '@/components/ui/spinner';
+import { useAuth } from '../hooks/useAuth';
+import { notify } from '@/lib/notify';
+import { cn } from '@/lib/utils';
+
+const NAV = [
+  { to: '/admin/dashboard', label: '仪表盘', icon: LayoutDashboard },
+  { to: '/admin/boards', label: '板块管理', icon: FolderKanban },
+  { to: '/admin/posts', label: '帖子管理', icon: FileText },
+  { to: '/admin/comments', label: '评论管理', icon: MessageSquare },
+  { to: '/admin/users', label: '用户管理', icon: Users },
+  { to: '/admin/settings', label: '系统设置', icon: Settings },
+];
+
+/** React 管理后台布局，与前台 SPA 风格统一 */
+export default function AdminLayout() {
+  const { user, loading } = useAuth();
+  const nav = useNavigate();
+
+  useEffect(() => {
+    if (loading) return;
+    if (!user) {
+      nav('/login');
+      return;
+    }
+    if (user.role !== 'admin') {
+      notify.warning('需要管理员权限');
+      nav('/');
+    }
+  }, [user, loading, nav]);
+
+  if (loading) {
+    return <div className="flex justify-center py-24"><Spinner size="lg" /></div>;
+  }
+  if (!user || user.role !== 'admin') return null;
+
+  return (
+    <div className="admin-shell">
+      <header className="admin-topbar">
+        <div className="admin-topbar-brand">
+          <div className="admin-topbar-mark">姜</div>
+          <div>
+            <div className="admin-topbar-title">姜十三论坛</div>
+            <div className="admin-topbar-sub">管理后台</div>
+          </div>
+        </div>
+        <div className="admin-topbar-actions">
+          <button type="button" className="admin-link-btn" onClick={() => nav('/')}>
+            <ArrowLeft size={16} />
+            返回论坛
+          </button>
+          <span className="admin-topbar-user">{user.nickname}</span>
+        </div>
+      </header>
+
+      <div className="admin-body">
+        <aside className="admin-sidebar">
+          {NAV.map(({ to, label, icon: Icon }) => (
+            <NavLink
+              key={to}
+              to={to}
+              className={({ isActive }) => cn('admin-nav-item', isActive && 'active')}
+            >
+              <Icon size={16} />
+              {label}
+            </NavLink>
+          ))}
+        </aside>
+        <main className="admin-main">
+          <Outlet />
+        </main>
+      </div>
+    </div>
+  );
+}
+
+/** 管理页通用权限守卫 */
+export function useAdminGuard() {
+  const { user, loading } = useAuth();
+  const nav = useNavigate();
+
+  useEffect(() => {
+    if (loading) return;
+    if (!user) nav('/login');
+    else if (user.role !== 'admin') {
+      notify.warning('需要管理员权限');
+      nav('/');
+    }
+  }, [user, loading, nav]);
+
+  return { user, loading, ready: !loading && !!user && user.role === 'admin' };
+}