feat: 添加页面管理系统和首页/关卡页面

- 实现 ViewManager 单例页面管理器，支持页面注册、打开、关闭、缓存
- 实现 BaseView 页面基类，提供统一的页面生命周期
- 添加 PageHome 首页，包含开始游戏按钮跳转功能
- 添加 PageLevel 关卡页面，继承 BaseView
- 更新 PageLoading 支持进度条显示和页面预加载
- 添加相关图片资源和预制体

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/PageLoading.prefab

@@ -32,10 +32,13 @@
       },
       {
         "__id__": 26
+      },
+      {
+        "__id__": 28
       }
     ],
     "_prefab": {
-      "__id__": 28
+      "__id__": 30
     },
     "_lpos": {
       "__type__": "cc.Vec3",
@@ -567,12 +570,51 @@
     "__prefab": {
       "__id__": 27
     },
+    "progressBar": {
+      "__id__": 21
+    },
     "_id": ""
   },
   {
     "__type__": "cc.CompPrefabInfo",
     "fileId": "035jtIcBFAXpr+rMnkwU6G"
   },
+  {
+    "__type__": "cc.Widget",
+    "_name": "",
+    "_objFlags": 0,
+    "__editorExtras__": {},
+    "node": {
+      "__id__": 1
+    },
+    "_enabled": true,
+    "__prefab": {
+      "__id__": 29
+    },
+    "_alignFlags": 45,
+    "_target": null,
+    "_left": 0,
+    "_right": 0,
+    "_top": 0,
+    "_bottom": 0,
+    "_horizontalCenter": 0,
+    "_verticalCenter": 0,
+    "_isAbsLeft": true,
+    "_isAbsRight": true,
+    "_isAbsTop": true,
+    "_isAbsBottom": true,
+    "_isAbsHorizontalCenter": true,
+    "_isAbsVerticalCenter": true,
+    "_originalWidth": 1080,
+    "_originalHeight": 2160,
+    "_alignMode": 2,
+    "_lockFlags": 0,
+    "_id": ""
+  },
+  {
+    "__type__": "cc.CompPrefabInfo",
+    "fileId": "08Ke4fRRVPz6x9lHDiuWL/"
+  },
   {
     "__type__": "cc.PrefabInfo",
     "root": {