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/screens/checkout_screen.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/material.dart';
+
+class CheckoutScreen extends StatelessWidget {
+  const CheckoutScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Checkout')),
+      body: Center(
+        child: ConstrainedBox(
+          constraints: const BoxConstraints(maxWidth: 640),
+          child: Card(
+            margin: const EdgeInsets.all(20),
+            child: Padding(
+              padding: const EdgeInsets.all(20),
+              child: Column(
+                mainAxisSize: MainAxisSize.min,
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: const [
+                  Text(
+                    'Checkout (Skeleton)',
+                    style: TextStyle(fontSize: 24, fontWeight: FontWeight.bold),
+                  ),
+                  SizedBox(height: 10),
+                  Text('This page is intentionally a placeholder for:'),
+                  SizedBox(height: 8),
+                  Text('- Shipping address collection'),
+                  Text('- Payment gateway integration'),
+                  Text('- Order review and confirmation flow'),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}