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/components/common/ImageGallery.tsx

@@ -33,6 +33,7 @@ import { MaterialCommunityIcons } from '@expo/vector-icons';
 import * as MediaLibrary from 'expo-media-library';
 import { File, Paths } from 'expo-file-system';
 import { spacing, borderRadius, fontSizes } from '../../theme';
+import { blurActiveElement } from '../../infrastructure/platform';
 
 const { width: SCREEN_WIDTH, height: SCREEN_HEIGHT } = Dimensions.get('window');
 
@@ -127,10 +128,7 @@ export const ImageGallery: React.FC<ImageGalleryProps> = ({
   // 打开/关闭时重置状态
   useEffect(() => {
     if (visible) {
-      if (Platform.OS === 'web') {
-        const active = (globalThis as any)?.document?.activeElement as { blur?: () => void } | undefined;
-        active?.blur?.();
-      }
+      blurActiveElement();
       setCurrentIndex(initialIndex);
       setShowControls(true);
       setError(false);