Implemented cart service with state tracker and linked to main layout

MidrandBookshop/Services/ShoppingCart/Models/Cart.cs

@@ -0,0 +1,18 @@
+namespace MidrandBookshop.Services.ShoppingCart.Models;
+
+public sealed class Cart
+{
+    public long? CustomerId { get; set; }
+
+    public long? OrderId { get; set; }
+
+    public decimal TotalPrice { get; set; }
+
+    public decimal TotalVat { get; set; }
+
+    public decimal VatRate { get; set; } = 0.15m;
+
+    public bool IsVatInclusive { get; set; } = true;
+
+    public IList<CartItem> Items { get; set; } = [];
+}