fix: restore data-language attribute in Shiki-highlighted code blocks

Shiki's codeToHtml() does not preserve the data-language attribute
from the original <pre> tag. The CSS language label depends on
pre[data-language]::before { content: attr(data-language); },
so the label disappears after Shiki highlighting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server.js

@@ -85,10 +85,12 @@ function highlightHtmlWithShiki(html, themeName = 'github-light') {
     // Try Shiki highlight
     if (lang && shikiHighlighter.getLoadedLanguages().includes(lang)) {
       try {
-        const highlighted = shikiHighlighter.codeToHtml(codeText
+        let highlighted = shikiHighlighter.codeToHtml(codeText
           .replace(/&lt;/g, '<').replace(/&gt;/g, '>').replace(/&amp;/g, '&').replace(/&quot;/g, '"'),
           { lang, theme: shikiTheme }
         );
+        // Restore data-language attribute for CSS language label display
+        highlighted = highlighted.replace(/<pre\s/, `<pre data-language="${lang}" `);
         return highlighted;
       } catch (e) {
         // Fallback to plain pre
@@ -167,9 +169,9 @@ app.post('/api/generate_pdf', async (req, res) => {
     const htmlSizeMB = (html.length / 1024 / 1024).toFixed(2);
     const imgCount = imageCount || 0;
     const imgSizeMB = totalImageSizeMB || 0;
-    
+
     console.log(`[PDF-GEN] [${getElapsed()}] 解析请求完成: HTML ${htmlSizeMB} MB, 图片 ${imgCount} 张 (${imgSizeMB} MB)`);
-    
+
     // 关键修复：移除所有图片的 loading="lazy" 属性
     // 问题：loading="lazy" 导致视口外的图片在 PDF 中不渲染
     // 解决方案：https://stackoverflow.com/questions/79156691/puppeteersharp-fails-to-display-base64-encoded-images-in-pdf-output