feat: 合并星图和聊天页面为 MainPage，响应式 GridRow 布局

- 新建 MainPage.ets，用 GridRow 响应式布局组合星图+聊天
- Index.ets 改为 2 Tab（TrulyMEM + 设置）
- GraphPage/ChatPage 接收 @Prop db 从父组件传入
- 恢复 README.md / README_EN.md 的特别鸣谢部分

harmonyos/trulymem-core/src/main/ets/GraphDatabase.ets

@@ -0,0 +1,21 @@
+import relationalStore from '@ohos.data.relationalStore';
+import { Context } from '@ohos.abilityAccessCtrl';
+
+const STORE_CONFIG: relationalStore.StoreConfig = {
+    name: 'trulymem.db',
+    securityLevel: relationalStore.SecurityLevel.S1
+};
+
+export class GraphDatabase {
+    private store?: relationalStore.RdbStore;
+    private context?: Context;
+
+    async init(context: Context) {
+        this.context = context;
+        this.store = await relationalStore.getRdbStore(context, STORE_CONFIG);
+    }
+
+    getStore(): relationalStore.RdbStore | undefined {
+        return this.store;
+    }
+}