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/util/BreakpointSystem.ets

@@ -0,0 +1,47 @@
+export enum WidthBreakpoint {
+  WIDTH_XS = 'xs',
+  WIDTH_SM = 'sm',
+  WIDTH_MD = 'md',
+  WIDTH_LG = 'lg',
+  WIDTH_XL = 'xl'
+}
+
+export interface BreakpointTypes<T> {
+  xs?: T;
+  sm: T;
+  md: T;
+  lg: T;
+  xl?: T;
+}
+
+export class BreakpointType<T> {
+  private xs: T;
+  private sm: T;
+  private md: T;
+  private lg: T;
+  private xl: T;
+
+  public constructor(param: BreakpointTypes<T>) {
+    this.xs = param.xs ?? param.sm;
+    this.sm = param.sm;
+    this.md = param.md;
+    this.lg = param.lg;
+    this.xl = param.xl ?? param.lg;
+  }
+
+  public getValue(currentBreakpoint: WidthBreakpoint): T {
+    if (currentBreakpoint === WidthBreakpoint.WIDTH_XS) {
+      return this.xs;
+    }
+    if (currentBreakpoint === WidthBreakpoint.WIDTH_SM) {
+      return this.sm;
+    }
+    if (currentBreakpoint === WidthBreakpoint.WIDTH_MD) {
+      return this.md;
+    }
+    if (currentBreakpoint === WidthBreakpoint.WIDTH_XL) {
+      return this.xl;
+    }
+    return this.lg;
+  }
+}