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/TermsOfServiceScreen.tsx

@@ -10,13 +10,15 @@ import {
   StyleSheet,
   ScrollView,
 } from 'react-native';
+import { StatusBar } from 'expo-status-bar';
 import { SafeAreaView, useSafeAreaInsets } from 'react-native-safe-area-context';
+import { useRouter } from 'expo-router';
 import {
   useAppColors,
   spacing,
   type AppColors,
 } from '../../theme';
-import { Text } from '../../components/common';
+import { Text, SimpleHeader } from '../../components/common';
 import { useResponsive, useResponsiveSpacing } from '../../hooks';
 
 // 内容最大宽度
@@ -213,12 +215,15 @@ export const TermsOfServiceScreen: React.FC = () => {
   const insets = useSafeAreaInsets();
   const { isMobile } = useResponsive();
   const responsivePadding = useResponsiveSpacing({ xs: 20, sm: 24, md: 28, lg: 32, xl: 40 });
+  const router = useRouter();
 
   // 底部间距，避免被 TabBar 遮挡
   const scrollBottomInset = isMobile ? 64 + 24 + insets.bottom + spacing.md : spacing.lg;
 
   return (
-    <SafeAreaView style={styles.container} edges={['bottom']}>
+    <SafeAreaView style={styles.container} edges={['top', 'bottom']}>
+      <StatusBar style="auto" />
+      <SimpleHeader title="用户协议" onBack={() => router.back()} />
       <ScrollView
         contentContainerStyle={[styles.scrollContent, { paddingHorizontal: responsivePadding, paddingBottom: scrollBottomInset }]}
         showsVerticalScrollIndicator={false}