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

graphql/fragments/order-cancelled.graphql

@@ -0,0 +1,13 @@
+fragment OrderCancelledWebhookPayload on OrderCancelled {
+  order {
+    id
+    number
+    userEmail
+    user {
+      email
+      firstName
+      lastName
+    }
+    status
+  }
+}