feat: complete saleor core extensions app with React email templates

- Order confirmation, shipped, and cancelled email templates
- Uses @react-email/components for professional HTML emails
- Sends admin and customer notifications
- Integrates with Resend for email delivery
- Webhook handlers for ORDER_CREATED, ORDER_FULFILLED, ORDER_CANCELLED
- Docker image optimized for production
- Persistent auth data storage via PVC

.env.example

@@ -0,0 +1,8 @@
+# Local development variables. When developped locally with Saleor inside docker, these can be set to:
+#
+# APP_IFRAME_BASE_URL = http://localhost:3000, so Dashboard on host can access iframe
+# APP_API_BASE_URL=http://host.docker.internal:3000 - so Saleor can reach App running on host, from the container.
+#
+# If developped with tunnels, set this empty, it will fallback to address the app is reached from (default port 3000).
+APP_IFRAME_BASE_URL=
+APP_API_BASE_URL=