Initialize project and update portal port configuration

Set default portal port to 8081, fix Dart build issue in cart screen, and update setup documentation.

Co-Authored-By: Oz <oz-agent@warp.dev>

lib/models/cart_item.dart

@@ -0,0 +1,10 @@
+import 'product.dart';
+
+class CartItem {
+  final Product product;
+  int quantity;
+
+  CartItem({required this.product, this.quantity = 1});
+
+  double get lineTotal => product.price * quantity;
+}