[Feat]: 导出图片增加设置图片的padding

### Describe the feature / 功能描述

1、graph.toDataUrl方法不支持指定padding
2、尝试图初始化时配置padding，导出图片也没有边距
3、导出图片前，改变画布大小，让元素居中，导出图片也没有边距

### Are you willing to contribute? / 是否愿意参与贡献？

✅ Yes / 是

Start at the graph.toDataUrl API and trace how export dimensions and canvas bounds are determined. Reproduce the three reported padding configurations, then verify that the requested padding appears around the exported graph without changing the graph layout.

Written by the indexing model from the issue text.