feat(widget): 为评论编辑器添加Markdown预览功能

- 引入 marked 和 DOMPurify 依赖以支持 Markdown 解析与安全渲染
- 在评论表单和回复编辑器中添加预览按钮，支持实时预览
- 新增预览区域样式，优化操作按钮布局间距
- 修复表单状态初始化问题，确保预览功能稳定

docs/widget/src/utils/markdown.js

@@ -0,0 +1,34 @@
+import { marked } from 'marked';
+import DOMPurify from 'dompurify';
+
+// 配置 marked
+try {
+  marked.setOptions({
+    gfm: true, // 启用 GitHub Flavored Markdown
+    breaks: true, // 启用换行符转 <br>
+  });
+} catch (e) {
+  console.error('Failed to configure marked:', e);
+}
+
+/**
+ * 渲染 Markdown 为 HTML，并进行净化
+ * @param {string} content Markdown 内容
+ * @returns {string} 净化后的 HTML
+ */
+export function renderMarkdown(content) {
+  if (!content) return '';
+  try {
+    const html = marked.parse(content);
+    // marked.parse can return a Promise if async is enabled, but we are using sync mode
+    // Just in case, handle potential Promise (though unlikely with current config)
+    if (html instanceof Promise) {
+      console.warn('marked.parse returned a Promise. Async markdown rendering is not fully supported in this sync flow.');
+      return '';
+    }
+    return DOMPurify.sanitize(html);
+  } catch (error) {
+    console.error('Markdown rendering error:', error);
+    return DOMPurify.sanitize(content); // Fallback to plain text (sanitized)
+  }
+}