refactor: CourseCanvas 使用默认画布尺寸，移除冗余 width/height 配置

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.config.ts

@@ -20,17 +20,6 @@ export const appConfig = {
 		name: env?.VITE_COURSE_NAME ?? "标准课程页示例",
 	},
 
-	/** 设计稿配置 */
-	design: {
-		width: 1920,
-		height: 1080,
-	},
-
-	/** 整体缩放配置 */
-	scale: {
-		max: 1,
-	},
-
 	/** 本地存储配置 */
 	storage: {
 		prefix: "yiban-course-template",