支持结账多轮协商与按实际消耗双比例结算

允许双方最多 6 轮修改结账方案；哈夫币按实际消耗与 buyer/seller 比例计价，打超从押金扣，押金不足禁止自动完结并引导人工争议。同步收紧结账明细展示密度。

backend/internal/modules/order/presenter.go

@@ -95,11 +95,27 @@ func toCheckoutAdminDTO(checkout model.OrderCheckout) CheckoutDTO {
 	platformFeeCent := checkout.PlatformFeeCent
 	renterRefundAmountCent := checkout.RenterRefundAmountCent
 	ownerIncomeAmountCent := checkout.OwnerIncomeAmountCent
+	roundCount := checkout.RoundCount
+	if roundCount <= 0 {
+		roundCount = 1
+	}
+	turn := checkout.Turn
+	if turn != checkoutTurnOwner && turn != checkoutTurnRenter {
+		if checkout.Status == checkoutStatusCountered {
+			turn = checkoutTurnRenter
+		} else {
+			turn = checkoutTurnOwner
+		}
+	}
 	return CheckoutDTO{
 		ID:                      checkout.ID,
 		OrderID:                 checkout.OrderID,
 		InitiatedBy:             checkout.InitiatedBy,
 		Status:                  checkout.Status,
+		RoundCount:              roundCount,
+		Turn:                    turn,
+		ProposedBy:              checkout.ProposedBy,
+		MaxRounds:               checkoutMaxRounds,
 		PriceRole:               "admin",
 		DisplayAmountCent:       rentAmountCent,
 		RentAmountCent:          &rentAmountCent,
@@ -112,6 +128,8 @@ func toCheckoutAdminDTO(checkout model.OrderCheckout) CheckoutDTO {
 		DepositDeductAmountCent: checkout.DepositDeductAmountCent,
 		RenterRefundAmountCent:  &renterRefundAmountCent,
 		OwnerIncomeAmountCent:   &ownerIncomeAmountCent,
+		ShortfallCent:           checkout.ShortfallCent,
+		OvershootAmountCent:     checkout.OvershootAmountCent,
 		Content:                 checkout.Content,
 		EvidenceURLS:            decodeStringList(checkout.EvidenceURLS),
 		OwnerAdjustmentReason:   checkout.OwnerAdjustmentReason,
@@ -125,10 +143,58 @@ func toCheckoutAdminDTO(checkout model.OrderCheckout) CheckoutDTO {
 
 func toCheckoutDTOForUser(checkout model.OrderCheckout, userID uint64, order model.RentalOrder) CheckoutDTO {
 	dto := toCheckoutAdminDTO(checkout)
+	refreshCheckoutSettlementDTO(&dto, order, checkout)
 	applyCheckoutPriceView(&dto, order, userID)
+	applyCheckoutActionFlags(&dto, checkout, order, userID)
 	return dto
 }
 
+// refreshCheckoutSettlementDTO 用当前公式重算展示字段（兼容旧单 shortfall/overshoot 为空）
+func refreshCheckoutSettlementDTO(dto *CheckoutDTO, order model.RentalOrder, checkout model.OrderCheckout) {
+	if dto == nil {
+		return
+	}
+	if checkout.Status != checkoutStatusSubmitted && checkout.Status != checkoutStatusCountered {
+		return
+	}
+	settlement := calculateCheckoutSettlement(order, checkout.ConsumableAmountCent, checkout.CoinConsumedM, checkout.DepositDeductAmountCent)
+	rent := settlement.ActualRentAmountCent
+	ownerRent := settlement.OwnerRentIncomeCent
+	platform := settlement.PlatformFeeCent
+	renterRefund := settlement.RenterRefundCent
+	ownerIncome := settlement.OwnerIncomeCent
+	dto.RentAmountCent = &rent
+	dto.OwnerRentAmountCent = &ownerRent
+	dto.PlatformFeeCent = &platform
+	dto.RenterRefundAmountCent = &renterRefund
+	dto.OwnerIncomeAmountCent = &ownerIncome
+	dto.DisplayAmountCent = rent
+	dto.ShortfallCent = settlement.ShortfallCent
+	dto.OvershootAmountCent = settlement.OvershootAmountCent
+}
+
+func applyCheckoutActionFlags(dto *CheckoutDTO, checkout model.OrderCheckout, order model.RentalOrder, userID uint64) {
+	if dto == nil {
+		return
+	}
+	negotiating := order.Status == orderStatusPendingCheckoutConfirm || order.Status == orderStatusPendingCheckoutAccept
+	if !negotiating || (checkout.Status != checkoutStatusSubmitted && checkout.Status != checkoutStatusCountered) {
+		return
+	}
+	turn := dto.Turn
+	round := dto.RoundCount
+	isOwner := userID == order.OwnerID
+	isRenter := userID == order.RenterID
+	if isOwner && turn == checkoutTurnOwner && order.Status == orderStatusPendingCheckoutConfirm {
+		dto.CanAccept = dto.ShortfallCent <= 0
+		dto.CanCounter = round < checkoutMaxRounds
+	}
+	if isRenter && turn == checkoutTurnRenter && order.Status == orderStatusPendingCheckoutAccept {
+		dto.CanAccept = dto.ShortfallCent <= 0
+		dto.CanCounter = round < checkoutMaxRounds
+	}
+}
+
 func applyOrderPriceView(dto *OrderDTO, order model.RentalOrder, userID uint64) {
 	if dto == nil {
 		return