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/screens/profile/UserScreen.tsx

@@ -5,22 +5,28 @@
  */
 
 import React from 'react';
-import { useLocalSearchParams } from 'expo-router';
+import { useLocalSearchParams, useRouter } from 'expo-router';
+import { SafeAreaView } from 'react-native-safe-area-context';
 import UserProfileScreen from './UserProfileScreen';
 import { useCurrentUser } from '../../stores/auth';
+import { SimpleHeader } from '../../components/common';
 
 export const UserScreen: React.FC = () => {
   const { userId: userIdParam } = useLocalSearchParams<{ userId?: string }>();
+  const router = useRouter();
   const userId = userIdParam || '';
   const currentUser = useCurrentUser();
   const isSelfProfile = !!currentUser?.id && currentUser.id === userId;
-  
+
   return (
-    <UserProfileScreen
-      mode={isSelfProfile ? 'self' : 'other'}
-      userId={isSelfProfile ? undefined : userId}
-      hasHeader
-    />
+    <SafeAreaView style={{ flex: 1 }} edges={['top', 'bottom']}>
+      <SimpleHeader title="用户主页" onBack={() => router.back()} />
+      <UserProfileScreen
+        mode={isSelfProfile ? 'self' : 'other'}
+        userId={isSelfProfile ? undefined : userId}
+        hasHeader={false}
+      />
+    </SafeAreaView>
   );
 };