refactor(ui): redesign TabBar to border-bottom indicator style

- Remove pill container styling with shadows and rounded corners
- Simplify tab styling and reduce font weights
- Change active indicator from 20px to span 50% width
- Add channel tag display to PostDetailScreen
- Simplify SafeAreaView edge handling in embedded screens

src/screens/home/HomeScreen.tsx

@@ -942,7 +942,7 @@ export const HomeScreen: React.FC = () => {
   // 搜索页面内嵌模式
   if (showSearch) {
     return (
-      <SafeAreaView style={styles.container} edges={['top', 'bottom']}>
+      <SafeAreaView style={styles.container} edges={['bottom']}>
         <StatusBar barStyle={statusBarStyle} backgroundColor={colors.background.paper} />
         <SearchScreen
           onBack={() => setShowSearch(false)}