feat(ui): redesign system message item with card-style layout and add title validation

- Refactor SystemMessageItem with modern card design, gradient colors, and improved visual hierarchy
- Add unread indicator, status badges, and enhanced avatar badges for message types
- Update CreatePostScreen to require title input and improve placeholder text
- Add Android back button handling for notification screens in embedded mode
- Replace manual shadow styles with theme shadow utilities across message screens

src/screens/message/MessageListScreen.tsx

@@ -22,6 +22,7 @@ import {
   Animated,
   TextInput,
   Keyboard,
+  BackHandler,
 } from 'react-native';
 import { SafeAreaView, useSafeAreaInsets } from 'react-native-safe-area-context';
 import { useNavigation, useIsFocused } from '@react-navigation/native';
@@ -283,6 +284,19 @@ export const MessageListScreen: React.FC = () => {
     }
   }, [isFocused]);
 
+  // 处理 Android 物理返回键 - 当显示通知页面时，返回键关闭通知页面
+  useEffect(() => {
+    const backHandler = BackHandler.addEventListener('hardwareBackPress', () => {
+      if (showNotifications) {
+        setShowNotifications(false);
+        return true; // 阻止默认行为
+      }
+      return false;
+    });
+
+    return () => backHandler.remove();
+  }, [showNotifications]);
+
   // 下拉刷新
   const onRefresh = useCallback(async () => {
     setRefreshing(true);