feat: Vike +data 服务端取数 / SEO Head 移植 / 404 错误页 / 服务端 DOM 模拟

- feedData.ts 聚合首页/板块/帖子详情数据，缺失内容 throw render(404)
- Go 侧 spa_meta/seo 的 head 生成逻辑移植到 src/ssr/head.ts
- _error 页复用站点壳渲染 404，+data 从 abortReason 恢复壳层数据
- linkedom 模拟服务端 DOM，DOMPurify 封装为 SSR 透传的 sanitizeHtml
- 新增 /api/home-boot 聚合端点（handler/home_boot.go）供 SSR 冷启动灌缓存

frontend/src/utils/postContent.ts

@@ -1,4 +1,4 @@
-import DOMPurify from 'dompurify';
+import { sanitizeHtml } from './purify';
 import type { Config } from 'dompurify';
 import { enhanceCodeBlocks } from './enhanceCodeBlocks';
 import { enhanceHeadingAnchors } from './postHeadings';
@@ -111,7 +111,7 @@ function extractGatedInnerHtml(el: Element, bodyClass: string, badgeClass: strin
 export function isHtmlEmpty(html: string): boolean {
   if (!html.trim()) return true;
   const doc = new DOMParser().parseFromString(
-    DOMPurify.sanitize(html, POST_CONTENT_PURIFY_CONFIG) as string,
+    sanitizeHtml(html, POST_CONTENT_PURIFY_CONFIG),
     'text/html',
   );
   // 有文本内容
@@ -130,7 +130,7 @@ export function renderPostContentHtml(
   if (!html.trim()) return '';
 
   const doc = new DOMParser().parseFromString(
-    DOMPurify.sanitize(html, POST_CONTENT_PURIFY_CONFIG) as string,
+    sanitizeHtml(html, POST_CONTENT_PURIFY_CONFIG),
     'text/html',
   );