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/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vike dev",
     "build": "vike build",
-    "preview": "vike preview"
+    "preview": "vike preview",
+    "start": "node server/index.mjs"
   },
   "dependencies": {
     "@dnd-kit/core": "^6.3.1",
@@ -35,6 +36,7 @@
     "autoprefixer": "^10.5.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "compression": "^1.8.1",
     "dayjs": "^1.11.13",
     "diff": "^9.0.0",
     "dompurify": "^3.4.10",