Adds a Send button to the order action extension and a corresponding
"send" op to /api/orders/:orderId/invoice. Generates the invoice on
demand if missing, then sends via the configured SMTP.
The order-action / order-block UI extensions are hosted on
extensions.shopifycdn.com and call our app via fetch(). Without CORS
headers the browser blocked the response. authenticate.admin already
returns a cors helper and handles OPTIONS preflight - wrap every
Response with it.
Gerhard Scheikl