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/cookie-signature/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "cookie-signature",
+  "version": "1.0.7",
+  "description": "Sign and unsign cookies",
+  "keywords": ["cookie", "sign", "unsign"],
+  "author": "TJ Holowaychuk <tj@learnboost.com>",
+  "license": "MIT",
+  "repository": { "type": "git", "url": "https://github.com/visionmedia/node-cookie-signature.git"},
+  "dependencies": {},
+  "devDependencies": {
+    "mocha": "*",
+    "should": "*"
+  },
+  "scripts": {
+    "test": "mocha --require should --reporter spec"
+  },
+  "main": "index"
+}