fix: 清理AI遗留代码，修复场景配置

- 删除三套旧菜单系统 (MainMenu.cs, MainMenuUI.cs, ModeSelectionUI.cs)
- Ground 添加 BoxCollider 并设置 tag 为 Ground
- UIManager.OnReturnClicked 改为通过 PageController 返回首页
- PageController 增加 ReturnToHome 方法，管理 BtnFixationBack 可见性
- 清理废弃场景 Experiment.unity / ModeSelection.unity 及历史截图

Doc/WebGLBuildSever/node_modules/side-channel-list/list.d.ts

@@ -0,0 +1,14 @@
+type ListNode<T, K> = {
+	key: K;
+	next: undefined | ListNode<T, K>;
+	value: T;
+};
+type RootNode<T, K> = {
+	next: undefined | ListNode<T, K>;
+};
+
+export function listGetNode<T, K>(list: RootNode<T, K>, key: ListNode<T, K>['key'], isDelete?: boolean): ListNode<T, K> | undefined;
+export function listGet<T, K>(objects: undefined | RootNode<T, K>, key: ListNode<T, K>['key']): T | undefined;
+export function listSet<T, K>(objects: RootNode<T, K>, key: ListNode<T, K>['key'], value: T): void;
+export function listHas<T, K>(objects: undefined | RootNode<T, K>, key: ListNode<T, K>['key']): boolean;
+export function listDelete<T, K>(objects: undefined | RootNode<T, K>, key: ListNode<T, K>['key']): ListNode<T, K> | undefined;