feat: 书库导入导出/图片变体/书籍搜索/小组件运行时等

新增：
- 书库导入导出（library_import/library_export）及测试
- 图片变体生成（image_variants）与响应式图片（responsiveImage）
- 书籍搜索（bookSearch）+ BookSearch/LibrarySearchGrid 组件
- 小组件运行时（widgetRuntime）与静态检查（widgetLint）
- 上传缓存中间件（upload_cache）与字体 CSS 提取脚本

其它：
- 后端 handlers/services 全量调整
- 前端页面、组件、库函数与配置更新

frontend/package.json

@@ -4,10 +4,12 @@
   "private": true,
   "scripts": {
     "dev": "next dev -p 3000",
+    "predev": "node scripts/extract-font-css.mjs",
     "build": "next build",
+    "postbuild": "node scripts/extract-font-css.mjs",
     "start": "next start -p 3000",
     "lint": "next lint",
-    "test": "node --experimental-strip-types --test lib/siteUrl.test.ts lib/jsonLd.test.ts lib/urlStyle.test.ts lib/mobileTabs.test.ts lib/headerNav.test.ts lib/widgetCode.test.ts lib/smartDiff.test.ts lib/dyEmoji.test.ts lib/entryBlocks.test.ts"
+    "test": "node --experimental-strip-types --test lib/siteUrl.test.ts lib/jsonLd.test.ts lib/urlStyle.test.ts lib/mobileTabs.test.ts lib/headerNav.test.ts lib/widgetCode.test.ts lib/widgetLint.test.ts lib/smartDiff.test.ts lib/dyEmoji.test.ts lib/entryBlocks.test.ts lib/bookSearch.test.ts"
   },
   "dependencies": {
     "hast-util-to-jsx-runtime": "^2.3.6",