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/routermanager/PageContext.ets

@@ -0,0 +1,54 @@
+export interface RouterParam {
+  routerName: string;
+  param?: object;
+}
+
+export interface IPageContext {
+  openPage(data: RouterParam, animated?: boolean): void;
+  popPage(animated?: boolean): void;
+  replacePage(data: RouterParam, animated?: boolean): void;
+}
+
+export class PageContext implements IPageContext {
+  private readonly pathStack: NavPathStack;
+
+  constructor() {
+    this.pathStack = new NavPathStack();
+  }
+
+  public get navPathStack(): NavPathStack {
+    return this.pathStack;
+  }
+
+  public replacePage(data: RouterParam, animated: boolean = true): void {
+    try {
+      this.pathStack.replacePath({ name: data.routerName, param: data.param }, animated);
+    } catch (err) {
+      console.error('Open Page ' + data.routerName + ' failed. ' + err.code + ' ' + err.message);
+    }
+  }
+
+  public openPage(data: RouterParam, animated: boolean = true): void {
+    try {
+      this.pathStack.pushPath({ name: data.routerName, param: data.param }, animated);
+    } catch (err) {
+      console.error('Open Page ' + data.routerName + ' failed. ' + err.code + ' ' + err.message);
+    }
+  }
+
+  public popPage(animated: boolean = true): void {
+    try {
+      this.pathStack.pop(animated);
+    } catch (err) {
+      console.error('Pop Page failed. ' + err.code + ' ' + err.message);
+    }
+  }
+
+  public popPageByIndex(index: number, animated: boolean = true): void {
+    this.pathStack.popToIndex(index, animated);
+  }
+
+  public clear(animated: boolean = true): void {
+    this.pathStack.clear(animated);
+  }
+}