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>

.env.example

@@ -0,0 +1,10 @@
+PORTAL_PORT=8081
+PORTAL_TITLE=Flutter E-Commerce Portal
+
+FIREBASE_API_KEY=replace-with-firebase-api-key
+FIREBASE_AUTH_DOMAIN=replace-with-project-id.firebaseapp.com
+FIREBASE_PROJECT_ID=replace-with-project-id
+FIREBASE_STORAGE_BUCKET=replace-with-project-id.appspot.com
+FIREBASE_MESSAGING_SENDER_ID=replace-with-sender-id
+FIREBASE_APP_ID=replace-with-app-id
+FIREBASE_MEASUREMENT_ID=