fix: convert-browser.js 画布尺寸和 convert-w2p.js 同步

多slide用容器尺寸，单slide用canvas（限制140cm）

web-to-ppt/convert-browser.js

@@ -506,11 +506,15 @@ async function convertToPptx(input, onProgress) {
     slides.push({ background: slideBg, objects });
   }
 
-  // 6. 画布尺寸
-  const slideW2 = canvas.width || (slideGroups.length > 0 ? slideGroups[0].slideRect.w : 1920);
-  const slideH2 = Math.min(canvas.height || 1080, MAX_H_IN * 72);
-  const sw = slideW2 / 72;
-  const sh = Math.min(slideH2 / 72, MAX_H_IN);
+  // 6. 画布尺寸：多slide用容器尺寸，单slide用canvas（限制140cm）
+  var sizeSource;
+  if (slideGroups.length > 1) {
+    sizeSource = slideGroups[0].slideRect;
+  } else {
+    sizeSource = { w: canvas.width || 1920, h: Math.min(canvas.height || 1080, MAX_H_IN * 72) };
+  }
+  const sw = sizeSource.w / 72;
+  const sh = Math.min(sizeSource.h / 72, MAX_H_IN);
 
   return {
     presentation: { layout: 'CUSTOM', slideWidth: sw, slideHeight: sh },