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/message/MessageListScreen.tsx

@@ -677,7 +677,7 @@ export const MessageListScreen: React.FC = () => {
   const renderSearchMode = () => (
     <View style={styles.searchModeContainer}>
       {/* 搜索头部 */}
-      <View style={[styles.searchHeader, { paddingTop: Math.max(spacing.sm, insets.top + spacing.xs) }]}>
+      <View style={[styles.searchHeader, { paddingTop: spacing.sm }]}>
         <View style={styles.searchShell}>
           <SearchBar
             value={searchText}