fix: resolve balance payment issue with coupon and fee calculation

This commit fixes the inconsistent calculation logic between order preview
and actual order creation, which caused balance payment failures when using
coupons.

Changes:
- Standardized fee calculation order in both preCreateOrderLogic and purchaseLogic
- Moved gift amount deduction after fee calculation to ensure correct total
- Removed premature gift amount deduction before transaction in purchaseLogic
- Gift amount is now only deducted within the database transaction

The calculation order is now unified:
1. Apply coupon discount
2. Calculate handling fee based on post-coupon amount
3. Deduct gift amount from total (including fee)

This ensures the preview amount matches the actual payment amount.

internal/logic/public/order/preCreateOrderLogic.go

@@ -117,18 +117,6 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		couponAmount = calculateCoupon(amount, couponInfo)
 	}
 	amount -= couponAmount
-
-	var deductionAmount int64
-	// Check user deduction amount
-	if u.GiftAmount > 0 {
-		if u.GiftAmount >= amount {
-			deductionAmount = amount
-			amount = 0
-		} else {
-			deductionAmount = u.GiftAmount
-			amount -= u.GiftAmount
-		}
-	}
 	var feeAmount int64
 	if req.Payment != 0 {
 		payment, err := l.svcCtx.PaymentModel.FindOne(l.ctx, req.Payment)
@@ -139,8 +127,19 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		// Calculate the handling fee
 		if amount > 0 {
 			feeAmount = calculateFee(amount, payment)
+			amount += feeAmount
+		}
+	}
+	// Calculate gift amount deduction after fee calculation
+	var deductionAmount int64
+	if u.GiftAmount > 0 && amount > 0 {
+		if u.GiftAmount >= amount {
+			deductionAmount = amount
+			amount = 0
+		} else {
+			deductionAmount = u.GiftAmount
+			amount -= u.GiftAmount
 		}
-		amount += feeAmount
 	}
 
 	resp = &types.PreOrderResponse{