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/SearchScreen.tsx

@@ -13,7 +13,7 @@ import {
   ScrollView,
   RefreshControl,
 } from 'react-native';
-import { SafeAreaView, useSafeAreaInsets } from 'react-native-safe-area-context';
+import { SafeAreaView } from 'react-native-safe-area-context';
 import { useRouter } from 'expo-router';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
 import { spacing, fontSizes, borderRadius, useAppColors, type AppColors } from '../../theme';
@@ -41,7 +41,6 @@ export const SearchScreen: React.FC<SearchScreenProps> = ({ onBack }) => {
   const colors = useAppColors();
   const styles = useMemo(() => createSearchScreenStyles(colors), [colors]);
   const router = useRouter();
-  const insets = useSafeAreaInsets();
   const { searchHistory: history, addSearchHistory, clearSearchHistory } = useUserStore();
   
   // 使用响应式 hook
@@ -494,13 +493,13 @@ export const SearchScreen: React.FC<SearchScreenProps> = ({ onBack }) => {
   };
 
   return (
-    <SafeAreaView style={styles.container} edges={['bottom']}>
+    <SafeAreaView style={styles.container} edges={['top', 'bottom']}>
       {/* 搜索输入框 */}
       <View
         style={[
           styles.searchHeader,
           {
-            paddingTop: Math.max(spacing.sm, insets.top + spacing.xs),
+            paddingTop: spacing.sm,
             paddingHorizontal: responsivePadding,
             paddingBottom: spacing.xs,
           },