refactor(scene): 重构影子节点结构并简化Shadow2D组件

- 移除动态创建影子节点的逻辑，改为在场景中预置Shadow节点
- 简化Shadow2D组件，移除target绑定和动态更新逻辑
- 统一所有影子节点的命名和结构，统一使用Shadow名称
- 调整影子节点位置，统一放在角色节点下作为子节点
- 移除所有动态影子参数配置，使用固定椭圆参数绘制影子

assets/scripts/Shadow2D.ts

@@ -1,60 +1,12 @@
-import { _decorator, Component, Node, Graphics, Color, UITransform } from 'cc';
-const { ccclass, property } = _decorator;
+import { _decorator, Component, Graphics, Color } from 'cc';
+const { ccclass } = _decorator;
 
 @ccclass('Shadow2D')
 export class Shadow2D extends Component {
-
-  @property(Node)
-  target: Node | null = null;   // 角色节点
-
-  @property
-  shadowWidth: number = 60;
-
-  @property
-  shadowHeight: number = 20;
-
-  @property
-  shadowAlpha: number = 80; // 0-255
-
-  private shadowNode: Node | null = null;
-
   start() {
-    if (!this.target) {
-      console.warn("Shadow2D: 未绑定角色节点 target");
-      return;
-    }
-
-    // 创建影子节点
-    this.shadowNode = new Node("Shadow");
-    let g = this.shadowNode.addComponent(Graphics);
-    this.target.addChild(this.shadowNode);
-
-    // 调整大小
-    let ui = this.shadowNode.addComponent(UITransform);
-    ui.setContentSize(this.shadowWidth, this.shadowHeight);
-
-    // 画椭圆
-    g.fillColor = new Color(0, 0, 0, this.shadowAlpha);
-    g.ellipse(0, 0, this.shadowWidth / 2, this.shadowHeight / 2);
+    let g = this.node.addComponent(Graphics);
+    g.fillColor = new Color(0, 0, 0, 100); // 半透明黑色
+    g.ellipse(0, 0, 40, 15);  // 椭圆（radiusX=40, radiusY=15）
     g.fill();
-
-    // 放到角色脚下
-    this.shadowNode.setPosition(0, -this.target.getComponent(UITransform).height / 2);
-  }
-
-  update(dt: number) {
-    if (!this.shadowNode || !this.target) return;
-
-    if (!this.target.active) {
-      this.shadowNode.active = false;
-      return;
-    }
-
-    // 始终保持在角色底部（比如角色动画高度变化时也能跟随）
-    let ui = this.target.getComponent(UITransform);
-
-    if (!ui) return;
-
-    this.shadowNode.setPosition(0, -ui.height / 2 + 10);
   }
 }