chore(utils): replace UI Avatars API with static default avatar

Remove external UI Avatars API dependency and use a static default avatar
URL instead. Simplifies avatar utility functions by eliminating dynamic
avatar generation.

internal/service/system_message_service.go

@@ -461,13 +461,13 @@ func (s *systemMessageServiceImpl) getActorInfo(ctx context.Context, operatorID
 	if s.userRepo != nil {
 		user, err := s.userRepo.GetByID(operatorID)
 		if err != nil {
-			return "用户", utils.GenerateDefaultAvatarURL("用户"), nil // 返回默认值，不阻断流程
+			return "用户", utils.DefaultAvatarURL, nil
 		}
 		avatar := utils.GetAvatarOrDefault(user.Username, user.Nickname, user.Avatar)
 		return user.Nickname, avatar, nil
 	}
 	// 如果没有用户仓储，返回默认值
-	return "用户", utils.GenerateDefaultAvatarURL("用户"), nil
+	return "用户", utils.DefaultAvatarURL, nil
 }
 
 // getPostTitle 获取帖子标题