fix: change htmlToUse from const to let for future reassignment compatibility

server.js

@@ -196,7 +196,7 @@ app.post('/api/generate_pdf', async (req, res) => {
     console.log(`[PDF-GEN] [${getElapsed()}] 已移除 loading="lazy" 属性`);
 
     // 使用处理后的 HTML
-    const htmlToUse = htmlToProcess;
+    let htmlToUse = htmlToProcess;
     
     const baseWaitTime = imgCount > 0 ? Math.min(500 + imgCount * 400, 6000) : 500;
     const sizeWaitTime = imgSizeMB > 0 ? Math.min(imgSizeMB * 100, 3000) : 0;