feat: 商品添加克重字段,订单添加通道费,运费按重量计算

- 商品模型: 添加weight字段(克重)
- 订单模型: 添加channel_fee字段(支付通道费)
- 后台商品管理: 添加克重输入框
- 后台订单详情: 显示通道费
- 订单创建: 根据商品重量计算运费,添加通道费计算
- 购物车: 根据商品重量计算运费,显示通道费

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/api/handlers/order.go

@@ -243,6 +243,7 @@ func (h *OrderHandler) Create(c *gin.Context) {
 
 	var subtotal float64
 	var totalQuantity int
+	var totalWeight float64
 	var orderItems []models.OrderItem
 	supplierMap := make(map[uint]bool)
 
@@ -280,6 +281,7 @@ func (h *OrderHandler) Create(c *gin.Context) {
 
 		subtotal += cart.Product.Price * float64(cart.Quantity)
 		totalQuantity += cart.Quantity
+			totalWeight += cart.Product.Weight * float64(cart.Quantity)
 		orderItems = append(orderItems, models.OrderItem{
 			ProductID: cart.ProductID,
 			Quantity:  cart.Quantity,
@@ -298,7 +300,7 @@ func (h *OrderHandler) Create(c *gin.Context) {
 		break
 	}
 
-	var shippingFeeFirstWeight, shippingFeePerGram, serviceFeeRate, taxRate float64
+	var shippingFeeFirstWeight, shippingFeePerGram, serviceFeeRate, taxRate, channelFeeRate float64
 	var setting models.SystemSetting
 	if err := utils.DB.Where("`key` = ?", "shipping_fee_first_weight").First(&setting).Error; err == nil {
 		shippingFeeFirstWeight, _ = strconv.ParseFloat(setting.Value, 64)
@@ -312,17 +314,23 @@ func (h *OrderHandler) Create(c *gin.Context) {
 	if err := utils.DB.Where("`key` = ?", "tax_rate").First(&setting).Error; err == nil {
 		taxRate, _ = strconv.ParseFloat(setting.Value, 64)
 	}
+	if err := utils.DB.Where("`key` = ?", "payment_channel_fee_rate").First(&setting).Error; err == nil {
+		channelFeeRate, _ = strconv.ParseFloat(setting.Value, 64)
+	}
 
-	shippingFee := shippingFeeFirstWeight
-	if subtotal >= 99 || shippingFeeFirstWeight == 0 {
-		shippingFee = 0
-	} else if totalQuantity > 500 {
-		shippingFee += shippingFeePerGram * float64(totalQuantity-500)
+	// 根据商品重量计算运费
+	shippingFee := 0.0
+	if subtotal < 99 && shippingFeeFirstWeight > 0 {
+		shippingFee = shippingFeeFirstWeight
+		if totalWeight > 500 {
+			shippingFee += shippingFeePerGram * (totalWeight - 500)
+		}
 	}
 
 	serviceFee := subtotal * serviceFeeRate / 100
 	tax := subtotal * taxRate / 100
-	totalAmount := subtotal + shippingFee + serviceFee + tax
+	channelFee := subtotal * channelFeeRate / 100
+	totalAmount := subtotal + shippingFee + serviceFee + tax + channelFee
 
 	order := models.Order{
 		UserID:            userID,
@@ -331,6 +339,7 @@ func (h *OrderHandler) Create(c *gin.Context) {
 		ShippingFee:       shippingFee,
 		ServiceFee:        serviceFee,
 		Tax:               tax,
+		ChannelFee:        channelFee,
 		TotalAmount:       totalAmount,
 		Status:            models.OrderStatusPendingPayment,
 		ShippingAddressID: &req.ShippingAddressID,