feat(emails): implement transactional email system with Resend

- Add Resend email integration with @react-email/render
- Create email templates: OrderConfirmation, OrderShipped, OrderCancelled, OrderPaid
- Implement webhook handler for ORDER_CREATED and other events
- Add multi-language support for customer emails
- Admin emails in English with order details
- Update checkout page with auto-scroll on order completion
- Configure DASHBOARD_URL environment variable

src/i18n/messages/de.json

@@ -345,6 +345,7 @@
     "emailRequired": "Erforderlich für Bestellbestätigung",
     "phoneRequired": "Erforderlich für Lieferkoordination",
     "shippingAddress": "Lieferadresse",
+    "shippingMethod": "Versandart",
     "country": "Land",
     "firstName": "Vorname",
     "lastName": "Nachname",