feat: SVG 图标序列化为图片（第三版）

- 不修改 backgroundUrls（const）
- 节点创建后直接修改 node.backgroundImages
- XMLSerializer + btoa 序列化 SVG

web-to-ppt/capture.js

@@ -985,7 +985,7 @@
     pushCaptureChild(children, afterNode);
 
     const type = element.tagName === "IMG" || backgroundUrls.length ? "RECTANGLE" : "FRAME";
-    return {
+    const node = {
       id: `node-${++nodeCounter}`,
       type,
       tag: element.tagName,
@@ -1005,6 +1005,19 @@
       backgroundImages: backgroundUrls,
       children
     };
+
+    // SVG 图标 → 序列化为 data URL 图片
+    if ((element.tagName === "SVG" || element.tagName === "svg") && !node.backgroundImages.length) {
+      try {
+        const svgClone = element.cloneNode(true);
+        if (!svgClone.getAttribute("xmlns")) svgClone.setAttribute("xmlns", "http://www.w3.org/2000/svg");
+        const svgStr = new XMLSerializer().serializeToString(svgClone);
+        node.backgroundImages = ["data:image/svg+xml;base64," + btoa(unescape(encodeURIComponent(svgStr)))];
+        node.type = "RECTANGLE";
+      } catch (e) {}
+    }
+
+    return node;
   }
 
   function normalizeAssetUrl(url) {