feat: 页脚分栏支持最多三栏并行（品牌+3栏目4列自适应布局，编辑器与后端校验上限 3）

frontend/src/components/Footer.jsx

@@ -29,20 +29,25 @@ export default function Footer({ settings = {}, version = '' }) {
   const desc = settings.footer_desc || settings.site_description || '';
 
   if (style === 'columns') {
-    // 栏目来源：后台 footer_columns 配置（非空优先）；空则回退硬编码导航
+    // 栏目来源：后台 footer_columns 配置（非空优先，最多取前 3 栏）；空则回退硬编码三栏导航
     const hardcodedCols = [
       { title: '导航', links: [
         { label: '首页', url: '/' },
         { label: '博客', url: '/blog.html' },
         { label: '论坛', url: '/forum.html' },
       ] },
-      { title: '其他', links: [
-        { label: '管理后台', url: '/admin', fullPage: true },
+      { title: '阅读', links: [
+        { label: '归档', url: '/archive.html' },
+        { label: 'RSS 订阅', url: '/feed.xml' },
+      ] },
+      { title: '账号', links: [
         { label: '个人中心', url: '/profile.html' },
+        { label: '管理后台', url: '/admin', fullPage: true },
+        { label: '登录', url: '/login.html' },
       ] },
     ];
     const customCols = parseFooterColumns(settings.footer_columns);
-    const cols = customCols.length > 0 ? customCols : hardcodedCols;
+    const cols = (customCols.length > 0 ? customCols : hardcodedCols).slice(0, 3);
 
     /** 栏目链接：站内路径走 SPA Link（整页跳转项除外）；http(s) 外链走 /out 确认页 */
     const colLink = (l, i) => {