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/Index.ets

@@ -3,3 +3,6 @@ export { GraphDatabase, RecallEntity, TimeRangeParams } from './src/main/ets/mod
 export { GraphMemoryService } from './src/main/ets/service/GraphMemoryService';
 export { AIAgentService, ChatMessage, AgentResponse } from './src/main/ets/service/AIAgentService';
 export { ImmersiveTabNavigation } from './src/main/ets/component/ImmersiveTabNavigation';
+export { PageContext, RouterParam, IPageContext } from './src/main/ets/routermanager/PageContext';
+export { BreakpointType, BreakpointTypes, WidthBreakpoint } from './src/main/ets/util/BreakpointSystem';
+export { BaseViewModel, VMEvent } from './src/main/ets/viewmodel/BaseViewModel';