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.

app/_layout.tsx

@@ -19,7 +19,7 @@ import {
   usePaperThemeFromStore,
   useResolvedColorScheme,
 } from '../src/theme';
-import { AppBackButton } from '../src/components/common';
+
 import AppPromptBar from '../src/components/common/AppPromptBar';
 import AppDialogHost from '../src/components/common/AppDialogHost';
 import { installAlertOverride } from '@/services/ui';
@@ -232,7 +232,6 @@ function APKUpdateBootstrap() {
 
 function ThemedStack() {
   const router = useRouter();
-  const colors = useAppColors();
   const resolved = useResolvedColorScheme();
 
   return (
@@ -250,49 +249,25 @@ function ThemedStack() {
           <Stack.Screen
             name="terms"
             options={{
-              headerShown: true,
-              title: '用户协议',
-              headerStyle: { backgroundColor: colors.background.paper },
-              headerTintColor: colors.text.primary,
-              headerShadowVisible: false,
-              headerBackVisible: false,
-              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+              headerShown: false,
             }}
           />
           <Stack.Screen
             name="privacy"
             options={{
-              headerShown: true,
-              title: '隐私政策',
-              headerStyle: { backgroundColor: colors.background.paper },
-              headerTintColor: colors.text.primary,
-              headerShadowVisible: false,
-              headerBackVisible: false,
-              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+              headerShown: false,
             }}
           />
           <Stack.Screen
             name="post/[postId]"
             options={{
-              headerShown: true,
-              title: '',
-              // 与 PostDetailScreen 的 SafeAreaView（background.default）同色，避免出现 header/内容之间的色差线
-              headerStyle: { backgroundColor: colors.background.default },
-              headerTintColor: colors.text.primary,
-              headerShadowVisible: false,
-              headerBackVisible: false,
-              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+              headerShown: false,
             }}
           />
           <Stack.Screen
             name="user/[userId]"
             options={{
-              headerShown: true,
-              title: '用户主页',
-              headerStyle: { backgroundColor: colors.background.paper },
-              headerTintColor: colors.text.primary,
-              headerBackVisible: false,
-              headerLeft: () => <AppBackButton onPress={() => router.back()} />,
+              headerShown: false,
             }}
           />
         </Stack>