Mirrors the layout from data/mail_template.png:
- Company name + greeting headline
- Body referencing the invoice number
- Inline logo (cid:invoice-logo) attached automatically
- Footer with mailto + website links
New template vars: {{shopEmail}}, {{shopWebsite}}.
Settings UI prefills empty fields with the defaults so users see and
can tweak them without losing the fallback.
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.
Gerhard Scheikl