fix: 添加购物车时检查购物资格

- 后端 CartHandler.Add 新增购物资格检查
- 后端新增库存检查和最大购买数量检查
- 前端商品详情页添加资格不足提示和视觉反馈
- 显示用户当前资格和所需资格总数

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

backend/internal/api/handlers/order.go

@@ -34,11 +34,61 @@ func (h *CartHandler) Add(c *gin.Context) {
 		return
 	}
 
-	var cart models.Cart
-	result := utils.DB.Where("user_id = ? AND product_id = ?", userID, req.ProductID).First(&cart)
+	// 获取商品信息
+	var product models.Product
+	if err := utils.DB.First(&product, req.ProductID).Error; err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "Product not found"})
+		return
+	}
+
+	// 检查商品是否上架
+	if !product.IsActive {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Product is not available"})
+		return
+	}
+
+	// 检查购物资格
+	if product.RequireCredit {
+		var user models.User
+		utils.DB.First(&user, userID)
+		totalCreditCost := product.CreditCost * req.Quantity
+		if user.PurchaseCredits < totalCreditCost {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "Insufficient purchase credits. Required: " + strconv.Itoa(totalCreditCost) + ", Available: " + strconv.Itoa(user.PurchaseCredits)})
+			return
+		}
+	}
+
+	// 检查库存
+	var inventory models.Inventory
+	if err := utils.DB.Where("product_id = ?", req.ProductID).First(&inventory).Error; err == nil {
+		if inventory.Quantity < req.Quantity {
+			c.JSON(http.StatusBadRequest, gin.H{"error": "Insufficient stock. Available: " + strconv.Itoa(inventory.Quantity)})
+			return
+		}
+	}
+
+	// 检查购物车中是否已存在该商品
+	var existingCart models.Cart
+	var totalQuantity int
+	result := utils.DB.Where("user_id = ? AND product_id = ?", userID, req.ProductID).First(&existingCart)
 	if result.Error == nil {
-		cart.Quantity += req.Quantity
-		utils.DB.Save(&cart)
+		totalQuantity = existingCart.Quantity + req.Quantity
+	} else {
+		totalQuantity = req.Quantity
+	}
+
+	// 检查最大购买数量
+	if product.MaxPurchase != nil && *product.MaxPurchase > 0 && totalQuantity > *product.MaxPurchase {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Maximum purchase quantity is " + strconv.Itoa(*product.MaxPurchase)})
+		return
+	}
+
+	// 添加或更新购物车
+	var cart models.Cart
+	if result.Error == nil {
+		existingCart.Quantity = totalQuantity
+		utils.DB.Save(&existingCart)
+		cart = existingCart
 	} else {
 		cart = models.Cart{
 			UserID:    userID,