fix: AIAgentService getContext(this) 改为外部传入，Index 添加 aboutToDisappear 释放 display 监听

- AIAgentService: 添加 appContext 字段，构造函数接收 Context 参数
- ChatPage: 创建 AIAgentService 时传入 getContext(this)
- Index: 添加 displayCallback 字段和 aboutToDisappear 释放监听

entry/src/main/ets/pages/Index.ets

@@ -8,11 +8,18 @@ import { ImmersiveTabNavigation } from '../components/ImmersiveTabNavigation';
 struct Index {
   @State currentIndex: number = 0;
   private db: GraphDatabase = new GraphDatabase();
+  private displayCallback?: (data: display.Display) => void;
 
   aboutToAppear() {
     this.db.init(getContext(this));
   }
 
+  aboutToDisappear() {
+    if (this.displayCallback) {
+      display.off('change', this.displayCallback);
+    }
+  }
+
   @Builder
   tabContentBuilder() {
     Column() {