refactor: 应用参考工程代码模式，优化启动流程和架构

- 新增 SplashPage：启动页，2秒后跳转到 Index
- 新增 MainPage：独立主页组件，含 Tab 导航 + 宽屏/窄屏自适应
- 简化 Index.ets：纯路由入口，初始化数据库，加载 MainPage
- EntryAbility：加载 SplashPage 而非直接加载 Index
- 新增 PageContext：NavPathStack 路由管理（参考工程模式）
- 新增 BreakpointSystem：响应式断点系统 xs/sm/md/lg/xl
- 新增 BaseViewModel：ViewModel 基类，含 attach/detach/dispose 生命周期
- 更新 common/Index.ets 导出新模块
- 更新 main_pages.json 注册 SplashPage 和 MainPage

common/src/main/ets/viewmodel/BaseViewModel.ets

@@ -0,0 +1,52 @@
+import { BreakpointType, WidthBreakpoint } from '../util/BreakpointSystem';
+
+export interface VMEvent {
+}
+
+export class BaseViewModel {
+  protected isAttached: boolean = false;
+  protected isDisposed: boolean = false;
+  protected currentBreakpoint: WidthBreakpoint = WidthBreakpoint.WIDTH_MD;
+
+  attach(): void {
+    if (this.isAttached) {
+      return;
+    }
+    this.isAttached = true;
+    this.onAttach();
+  }
+
+  detach(): void {
+    if (!this.isAttached) {
+      return;
+    }
+    this.isAttached = false;
+    this.onDetach();
+  }
+
+  dispose(): void {
+    if (this.isDisposed) {
+      return;
+    }
+    this.isDisposed = true;
+    this.detach();
+    this.onDispose();
+  }
+
+  protected onAttach(): void {
+  }
+
+  protected onDetach(): void {
+  }
+
+  protected onDispose(): void {
+  }
+
+  public get attached(): boolean {
+    return this.isAttached;
+  }
+
+  public get disposed(): boolean {
+    return this.isDisposed;
+  }
+}