refactor: use unified util to create plugin canvas

[Aarebecca]

• 提供统一的 createPluginCanvas 工具函数为插件创建独立画布
• 图例插件绘制于独立画布中，不受相机缩放、移动影响

由于绘制到其他画布，因此截图中无法显示出来。
后续考虑提供特殊 hook，在截图、导出图片时从插件读取额外元素

---

• Provide a unified 'createPluginCanvas' utility function to create a standalone canvas for the plugin
• The legend plugin is drawn in a standalone canvas and is not affected by camera zoom or movement

Cannot be shown in screenshots because they are drawn to a different canvas.
Consider providing a special hook to read additional elements from the plugin when taking screenshots and exporting images