feat: bulk update accounts pre-check mixed channel risk with confirm dialog

- Move mixed channel check before any DB writes in BulkUpdateAccounts
- Return 409 from BulkUpdate handler for MixedChannelError
- Add ConfirmDialog to BulkEditAccountModal for mixed channel warning
- Update mixed channel warning message to Chinese

backend/internal/handler/admin/account_handler.go

@@ -1122,6 +1122,14 @@ func (h *AccountHandler) BulkUpdate(c *gin.Context) {
 		SkipMixedChannelCheck: skipCheck,
 	})
 	if err != nil {
+		var mixedErr *service.MixedChannelError
+		if errors.As(err, &mixedErr) {
+			c.JSON(409, gin.H{
+				"error":   "mixed_channel_warning",
+				"message": mixedErr.Error(),
+			})
+			return
+		}
 		response.ErrorFrom(c, err)
 		return
 	}