refactor: use unified util to create plugin canvas

[Aarebecca]

• [x] 提供统一的 createPluginCanvas 工具函数为插件创建独立画布
• [x] 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

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• [x] Provide a unified 'createPluginCanvas' utility function to create a standalone canvas for the plugin
• [x] 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