refactor(ui): implement custom header system and unify screen layouts

Refactor the navigation and header strategy by replacing native stack headers with a custom `SimpleHeader` component across most profile and detail screens. This provides a more consistent UI/UX and better control over layout behavior.

- Implement `SimpleHeader` component in `src/components/common`.
- Disable native header rendering in `app/(app)/(tabs)/profile/_layout.tsx` and `app/_layout.tsx`.
- Update profile sub-screens to use `SimpleHeader` and `StatusBar` from `expo-status-bar`.
- Refactor `PostDetailScreen` to use a custom header implementation for better integration with the post author information.
- Update `UserScreen` to wrap content in `SafeAreaView` with the new header.
- Adjust `AppBackButton` to support transparent backgrounds.

src/components/common/index.ts

@@ -18,6 +18,7 @@ export { default as ResponsiveGrid } from './ResponsiveGrid';
 export { default as ResponsiveStack, HStack, VStack } from './ResponsiveStack';
 export { default as AdaptiveLayout, SidebarLayout } from './AdaptiveLayout';
 export { default as AppBackButton } from './AppBackButton';
+export { SimpleHeader } from './SimpleHeader';
 export { default as PagerView } from './PagerView';
 
 // 图片相关组件