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

tests/unit/contactMail.test.ts

@@ -0,0 +1,70 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { CONTACT_RECIPIENT, contactMail, mailSendUrl } from "../../server/utils/contactMail.ts";
+
+// components/ContactForm.vue posts these fields; it sends "Kontaktanfrage über Website" when the subject is left empty.
+const form = {
+	name: "Erika Mustermann",
+	email: "erika@example.org",
+	subject: "Notizbuch mit Firmenlogo",
+	message: "Hallo,\nkönnt ihr 50 Stück liefern?"
+};
+
+const required = { ok: false, message: "Name, E-Mail und Nachricht sind erforderlich" };
+
+test("sends to the mail service's /v1/send/message", () => {
+	assert.equal(mailSendUrl("http://mail:2222"), "http://mail:2222/v1/send/message");
+	assert.equal(mailSendUrl("http://mail:2222/"), "http://mail:2222/v1/send/message");
+});
+
+test("builds the body the mail service reads: to_email, subject and message", () => {
+	assert.deepEqual(contactMail(form), {
+		ok: true,
+		mail: {
+			to_email: "paperwork@muellerprints.de",
+			subject: "Kontaktanfrage: Notizbuch mit Firmenlogo",
+			message: [
+				"Name: Erika Mustermann",
+				"E-Mail: erika@example.org",
+				"Betreff: Notizbuch mit Firmenlogo",
+				"",
+				"Nachricht:",
+				"Hallo,\nkönnt ihr 50 Stück liefern?",
+				"",
+				"---",
+				"Diese Nachricht wurde über das Kontaktformular auf muellerprints.de gesendet."
+			].join("\n")
+		}
+	});
+	assert.equal(CONTACT_RECIPIENT, "paperwork@muellerprints.de");
+});
+
+test("fills in the subject when the form sends none", () => {
+	for (const subject of [undefined, "", "  "]) {
+		const result = contactMail({ ...form, subject });
+
+		assert.equal(result.ok && result.mail.subject, "Kontaktanfrage: Anfrage über Website");
+		assert.equal(result.ok && result.mail.message.split("\n")[2], "Betreff: Kontaktanfrage über Website");
+	}
+});
+
+test("keeps a line break in the subject from starting another mail header", () => {
+	const result = contactMail({ ...form, subject: "Frage\r\nBcc: someone@example.org\nX-Spam: yes" });
+
+	assert.equal(result.ok && result.mail.subject, "Kontaktanfrage: Frage Bcc: someone@example.org X-Spam: yes");
+});
+
+test("requires a name, an e-mail address and a message", () => {
+	for (const field of ["name", "email", "message"]) {
+		assert.deepEqual(contactMail({ ...form, [field]: undefined }), required, field);
+		assert.deepEqual(contactMail({ ...form, [field]: " " }), required, field);
+		assert.deepEqual(contactMail({ ...form, [field]: ["a"] }), required, field);
+	}
+	for (const body of [undefined, null, "name=Erika", [form]]) assert.deepEqual(contactMail(body), required);
+});
+
+test("rejects an invalid e-mail address", () => {
+	for (const email of ["erika", "erika@example", "erika @example.org", "@example.org"]) {
+		assert.deepEqual(contactMail({ ...form, email }), { ok: false, message: "Ungültige E-Mail-Adresse" }, email);
+	}
+});