feat: 添加异步导出功能，优化导出图片的下载流程

src/component/Canvas/CanvasEditor/managers/CanvasManager.js

@@ -810,7 +810,7 @@ export class CanvasManager {
    * @param {Boolean} options.restoreOpacityInRedGreen 红绿图模式下是否恢复透明度为1
    * @returns {String} 导出的图片数据URL
    */
-  exportImage(options = {}) {
+  async exportImage(options = {}) {
     if (!this.exportManager) {
       console.error("导出管理器未初始化，请确保已设置图层管理器");
       throw new Error("导出管理器未初始化");
@@ -852,7 +852,7 @@ export class CanvasManager {
         }
       }
 
-      return this.exportManager.exportImage(enhancedOptions);
+      return await this.exportManager.exportImage(enhancedOptions);
     } catch (error) {
       console.error("CanvasManager导出图片失败:", error);
       throw error;