应用实例一：三角形内角和动态验证
知识点：第十一章《三角形》—— 三角形内角和定理。
功能：学生可以拖动三角形的三个顶点任意改变三角形形状。应用实时计算并显示三个内角的度数，其和始终为180°。包含"折叠动画"按钮，演示将三个角拼成平角的过程。
@Entry @Component struct TriangleAngleSum { @State private p0x: number = 180 @State private p0y: number = 60 @State private p1x: number = 60 @State private p1y: number = 220 @State private p2x: number = 300 @State private p2y: number = 220 @State private angleA: number = 60 @State private angleB: number = 60 @State private angleC: number = 60 @State private isAnimating: boolean = false @State private animationProgress: number = 0 @State
