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/components/MarkdownEditor.tsx

@@ -83,6 +83,8 @@ type MarkdownEditorProps = {
   onUploadingChange?: (uploading: boolean) => void;
   /** 书级「全章节条目卡模式」：插入免壳模板、预览按隐式条目渲染 */
   entriesAuto?: boolean;
+  /** 图片上传时标记的 source（如 library_content），默认 post */
+  uploadSource?: string;
 };
 
 type EditorMode = "write" | "preview";
@@ -164,6 +166,7 @@ export default function MarkdownEditor({
   error,
   onUploadingChange,
   entriesAuto = false,
+  uploadSource,
 }: MarkdownEditorProps) {
   const taRef = useRef<HTMLTextAreaElement>(null);
   const urlStyle = useUrlStyle();
@@ -466,7 +469,7 @@ export default function MarkdownEditor({
     }
     setUploading(true);
     try {
-      const res = await apiUploadPostImage(file);
+      const res = await apiUploadPostImage(file, undefined, uploadSource);
       if (!res.url) {
         const msg = res.error || "上传失败";
         setUploadError(msg);
@@ -1268,12 +1271,13 @@ export default function MarkdownEditor({
             </div>
           ) : null}
         </div>
-        <div className="flex items-center gap-2 shrink-0">
+        <div className="j13-md-toolbar-end flex items-center gap-2 shrink-0">
           <div className="j13-md-mode" role="tablist" aria-label="编辑模式">
             <button
               type="button"
               role="tab"
               aria-selected={mode === "write"}
+              aria-label="编写模式"
               className={mode === "write" ? "is-on" : ""}
               onMouseDown={(e) => {
                 e.preventDefault();
@@ -1281,12 +1285,13 @@ export default function MarkdownEditor({
               }}
               onClick={() => setMode("write")}
             >
-              <Pencil size={12} /> 编写
+              <Pencil size={12} /> <span className="j13-md-mode-text">编写</span>
             </button>
             <button
               type="button"
               role="tab"
               aria-selected={mode === "preview"}
+              aria-label="预览模式"
               className={mode === "preview" ? "is-on" : ""}
               onMouseDown={(e) => {
                 e.preventDefault();
@@ -1294,7 +1299,7 @@ export default function MarkdownEditor({
               }}
               onClick={() => setMode("preview")}
             >
-              <Eye size={12} /> 预览
+              <Eye size={12} /> <span className="j13-md-mode-text">预览</span>
             </button>
           </div>
           {!toolNarrow && (