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

Assets/Scripts/UI/PageController.cs

@@ -126,18 +126,18 @@ public class PageController : MonoBehaviour
 
     #region 页面切换
 
-    private void ShowPage(Page page)
+        private void ShowPage(Page page)
     {
         _currentPage = page;
 
         if (_homePagePanel != null) _homePagePanel.SetActive(page == Page.Home);
         if (_modeSelectPanel != null) _modeSelectPanel.SetActive(page == Page.ModeSelect);
         if (_fixationPanel != null) _fixationPanel.SetActive(page == Page.Fixation);
+        if (_btnFixationBack != null) _btnFixationBack.gameObject.SetActive(page == Page.Fixation);
 
         if (_uiManager != null)
             _uiManager.enabled = (page == Page.Experiment);
     }
-
     #endregion
 
     #region 按钮回调
@@ -206,5 +206,11 @@ public class PageController : MonoBehaviour
         }
     }
 
+    public void ReturnToHome()
+    {
+        ShowPage(Page.Home);
+    }
+
+
     #endregion
 }