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/MessageListScreen.tsx

@@ -40,6 +40,7 @@ import {
 } from '../../theme';
 import { ConversationResponse, UserDTO, MessageResponse, extractTextFromSegments } from '../../types/dto';
 import { authService } from '../../services';
+import { blurActiveElement } from '../../infrastructure/platform';
 import { useUserStore, useAuthStore } from '../../stores';
 // 【新架构】使用MessageManager hooks（会话列表数据源自 MessageManager 游标同步）
 import {
@@ -138,9 +139,8 @@ export const MessageListScreen: React.FC = () => {
   const [actionMenuVisible, setActionMenuVisible] = useState(false);
 
   useEffect(() => {
-    if (actionMenuVisible && Platform.OS === 'web') {
-      const active = (globalThis as any)?.document?.activeElement as { blur?: () => void } | undefined;
-      active?.blur?.();
+    if (actionMenuVisible) {
+      blurActiveElement();
     }
   }, [actionMenuVisible]);
   const [scannerVisible, setScannerVisible] = useState(false);