fix(contact): send the contact form through the mail service's /v1/send/message

POST /api/contact posted { to, subject, body, replyTo } to ${mailApiUrl}/send,
which the mail service (libreshop/mail src/app.py) does not have, so every
message failed with a 500. It now posts { to_email, subject, message } to
POST /v1/send/message, still to paperwork@muellerprints.de, with the same
subject and text as before. The service sets no Reply-To; the sender's
address stays in the text. A line break in the subject is replaced by a
space, so it cannot start another mail header.

mailApiUrl was read from runtimeConfig without being declared there, so
NUXT_MAIL_API_URL could not set it and the fallback http://mail:2222 was
always used. It is declared now, with that default; mp's compose.yml sets
no NUXT_MAIL_API_URL.

contactMail and mailSendUrl (server/utils/contactMail.ts) are pure and tested.

Refs libretech/mp#71

nuxt.config.ts

@@ -29,6 +29,7 @@ export default defineNuxtConfig({
 		// Server-only (private) - never exposed to client
 		shopApiToken: "", // NUXT_SHOP_API_TOKEN
 		cmsInternalUrl: "http://cms:5555", // NUXT_CMS_INTERNAL_URL - internal docker network URL
+		mailApiUrl: "http://mail:2222", // NUXT_MAIL_API_URL - mail service (libreshop/mail) on the internal docker network
 		siteUrl: "https://muellerprints-paperwork.com", // NUXT_SITE_URL - for sitemap
 
 		// Public (client + server)