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/types/dto.ts

@@ -231,16 +231,18 @@ export type MessageSegment =
 
 // 消息响应
 export interface MessageResponse {
-  id: string;  // 雪花算法ID (使用string避免JavaScript精度丢失)
+  id: string;
   conversation_id: string;
-  sender_id: string;  // UUID字符串
-  seq: number;  // 消息序号
-  segments: MessageSegment[];  // 消息链
-  reply_to_id?: string;  // 被回复消息的ID（用于关联查找）
+  sender_id: string;
+  seq: number;
+  segments: MessageSegment[];
+  reply_to_id?: string;
   status: MessageStatus;
-  category?: string;  // 消息类别：chat, notification, announcement
+  category?: string;
   created_at: string;
   sender?: UserDTO;
+  is_system_notice?: boolean;
+  notice_content?: string;
 }
 
 // 会话响应