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>

docker-compose.yml

@@ -0,0 +1,20 @@
+name: flutter_ecommerce_portal
+
+services:
+  web:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    container_name: flutter_ecommerce_portal_web
+    ports:
+      - "${PORTAL_PORT:-8081}:80"
+    environment:
+      FIREBASE_API_KEY: ${FIREBASE_API_KEY}
+      FIREBASE_AUTH_DOMAIN: ${FIREBASE_AUTH_DOMAIN}
+      FIREBASE_PROJECT_ID: ${FIREBASE_PROJECT_ID}
+      FIREBASE_STORAGE_BUCKET: ${FIREBASE_STORAGE_BUCKET}
+      FIREBASE_MESSAGING_SENDER_ID: ${FIREBASE_MESSAGING_SENDER_ID}
+      FIREBASE_APP_ID: ${FIREBASE_APP_ID}
+      FIREBASE_MEASUREMENT_ID: ${FIREBASE_MEASUREMENT_ID}
+      PORTAL_TITLE: ${PORTAL_TITLE:-Flutter E-Commerce Portal}
+    restart: unless-stopped