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/services/cart_service.dart

@@ -0,0 +1,43 @@
+import 'package:flutter/foundation.dart';
+
+import '../models/cart_item.dart';
+import '../models/product.dart';
+
+class CartService extends ChangeNotifier {
+  final List<CartItem> _items = [];
+
+  List<CartItem> get items => List.unmodifiable(_items);
+
+  int get totalItems => _items.fold(0, (sum, item) => sum + item.quantity);
+
+  double get totalPrice =>
+      _items.fold(0.0, (sum, item) => sum + item.lineTotal);
+
+  void addProduct(Product product) {
+    final index = _items.indexWhere((item) => item.product.id == product.id);
+    if (index == -1) {
+      _items.add(CartItem(product: product));
+    } else {
+      _items[index].quantity += 1;
+    }
+    notifyListeners();
+  }
+
+  void removeProduct(String productId) {
+    _items.removeWhere((item) => item.product.id == productId);
+    notifyListeners();
+  }
+
+  void updateQuantity(String productId, int quantity) {
+    if (quantity < 1) return;
+    final index = _items.indexWhere((item) => item.product.id == productId);
+    if (index == -1) return;
+    _items[index].quantity = quantity;
+    notifyListeners();
+  }
+
+  void clear() {
+    _items.clear();
+    notifyListeners();
+  }
+}