refactor(gui): 更新加载界面资源及配置

- 删除未使用的背景资源mp2.png及其相关文件
- 更新umps.plist和umps.png资源文件
- 调整loading.prefab的激活状态和位置
- 优化资源引用和配置

assets/resources/gui/element/loading.prefab

@@ -25,7 +25,7 @@
         "__id__": 10
       }
     ],
-    "_active": false,
+    "_active": true,
     "_components": [
       {
         "__id__": 22
@@ -43,7 +43,7 @@
     "_lpos": {
       "__type__": "cc.Vec3",
       "x": 0,
-      "y": 640,
+      "y": 0,
       "z": 0
     },
     "_lrot": {
@@ -173,7 +173,7 @@
       "a": 255
     },
     "_spriteFrame": {
-      "__uuid__": "cb93c900-b440-4571-91d1-7da1636e3d73@2ef13",
+      "__uuid__": "d7d869bc-06aa-4876-806f-487e68b96780@8b868",
       "__expectedType__": "cc.SpriteFrame"
     },
     "_type": 1,
@@ -189,7 +189,7 @@
     "_isTrimmedMode": true,
     "_useGrayscale": false,
     "_atlas": {
-      "__uuid__": "cb93c900-b440-4571-91d1-7da1636e3d73",
+      "__uuid__": "d7d869bc-06aa-4876-806f-487e68b96780",
       "__expectedType__": "cc.SpriteAtlas"
     },
     "_id": ""
@@ -496,6 +496,7 @@
       "__id__": 0
     },
     "fileId": "5a9CMsVQhKP5Y+UJfTKPbx",
+    "instance": null,
     "targetOverrides": null,
     "nestedPrefabInstanceRoots": [
       {