refactor: 统一货币更新逻辑，修复奖励发放流程

1. 重构奖励发放代码，移除重复的金币持久化逻辑，统一使用updateGold方法
2. 新增钻石VM绑定字段与updateDiamond方法，实现金银币统一管理
3. 新增货币镜像同步方法syncCurrencyToView，维护持久层与视图层数据一致性
4. 修复两个奖励道具预制体的count_label绑定配置

assets/resources/gui/element/reward_item_exp.prefab

@@ -878,7 +878,9 @@
     "__prefab": {
       "__id__": 37
     },
-    "count_label": null,
+    "count_label": {
+      "__id__": 31
+    },
     "_id": ""
   },
   {
@@ -894,6 +896,7 @@
       "__id__": 0
     },
     "fileId": "47Sc4S8E5EfJ3uZtG97byB",
+    "instance": null,
     "targetOverrides": null
   }
 ]

assets/resources/gui/element/reward_item_gold.prefab

@@ -713,7 +713,9 @@
     "__prefab": {
       "__id__": 31
     },
-    "count_label": null,
+    "count_label": {
+      "__id__": 25
+    },
     "_id": ""
   },
   {
@@ -729,6 +731,7 @@
       "__id__": 0
     },
     "fileId": "10z6GxEehAebPrefUJTgkq",
+    "instance": null,
     "targetOverrides": null
   }
 ]