refactor(platform): extract blurActiveElement to infrastructure module

- Centralize duplicated Platform.OS === 'web' blur logic across 16 components
- Add blurActiveElement utility to src/infrastructure/platform module
- Optimize MessageBubble and ImageSegment with React.memo custom comparators
- Add useMemo for computed values in MessageSegmentsRenderer
- Update DTO types with is_system_notice and notice_content fields
- Fix keyboard dismissal order in useChatScreen handleDismiss
- Simplify userStore follow/unfollow state updates
- Remove unnecessary TypeScript type assertions throughout codebase

src/screens/message/GroupMembersScreen.tsx

@@ -22,6 +22,7 @@ import {
 import { SafeAreaView } from 'react-native-safe-area-context';
 import { useLocalSearchParams, useRouter } from 'expo-router';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
+import { blurActiveElement } from '../../infrastructure/platform';
 import { spacing, fontSizes, borderRadius, useAppColors, type AppColors } from '../../theme';
 import { useAuthStore } from '../../stores';
 import { groupService } from '../../services/groupService';
@@ -133,9 +134,8 @@ const GroupMembersScreen: React.FC = () => {
   const [newNickname, setNewNickname] = useState('');
 
   useEffect(() => {
-    if ((actionModalVisible || nicknameModalVisible) && Platform.OS === 'web') {
-      const active = (globalThis as any)?.document?.activeElement as { blur?: () => void } | undefined;
-      active?.blur?.();
+    if (actionModalVisible || nicknameModalVisible) {
+      blurActiveElement();
     }
   }, [actionModalVisible, nicknameModalVisible]);