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/home/HomeScreen.tsx

@@ -38,6 +38,7 @@ import { processPostUseCase } from '../../core/usecases/ProcessPostUseCase';
 import { SearchScreen } from './SearchScreen';
 import { CreatePostScreen } from '../create/CreatePostScreen';
 import * as hrefs from '../../navigation/hrefs';
+import { blurActiveElement } from '../../infrastructure/platform';
 
 const TABS = ['关注', '最新', '热门'];
 const TAB_ICONS = ['account-heart-outline', 'clock-outline', 'fire'];
@@ -203,9 +204,8 @@ export const HomeScreen: React.FC = () => {
   const [showCreatePost, setShowCreatePost] = useState(false);
 
   useEffect(() => {
-    if (showCreatePost && Platform.OS === 'web') {
-      const active = (globalThis as any)?.document?.activeElement as { blur?: () => void } | undefined;
-      active?.blur?.();
+    if (showCreatePost) {
+      blurActiveElement();
     }
   }, [showCreatePost]);