feat: message starter in the contact section

The visitor picks what they need and a prepared email types itself out; one
press opens it in their own mail app with subject and body filled in. The
mailto link is built by a tested pure function (src/domain/email.ts, spec
first) and only in the browser, so the address still never appears in the
page source.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/domain/email.spec.ts

@@ -1,5 +1,10 @@
 import { describe, expect, it } from "vitest";
-import { assembleEmail, parseEmailParts, spellOutEmail } from "./email";
+import {
+  assembleEmail,
+  mailtoHref,
+  parseEmailParts,
+  spellOutEmail,
+} from "./email";
 
 const parts = { user: "hello", domain: "mintel", tld: "me" } as const;
 
@@ -120,3 +125,47 @@ describe("spellOutEmail", () => {
     ).toBe("hello [at] mail [dot] site [dot] org");
   });
 });
+
+describe("mailtoHref", () => {
+  it("opens a message to the address with subject and body", () => {
+    expect(mailtoHref(parts, { subject: "Hello", body: "Hi" })).toBe(
+      "mailto:hello@mintel.me?subject=Hello&body=Hi",
+    );
+  });
+
+  it("encodes spaces as %20, not as plus signs", () => {
+    expect(mailtoHref(parts, { subject: "A website", body: "Hi Marc" })).toBe(
+      "mailto:hello@mintel.me?subject=A%20website&body=Hi%20Marc",
+    );
+  });
+
+  it("encodes line breaks so the mail app keeps the paragraphs", () => {
+    expect(mailtoHref(parts, { subject: "S", body: "one\n\ntwo" })).toBe(
+      "mailto:hello@mintel.me?subject=S&body=one%0D%0A%0D%0Atwo",
+    );
+  });
+
+  it("encodes characters that would break the link", () => {
+    expect(mailtoHref(parts, { subject: "Q&A: 50%?", body: "a=b #1" })).toBe(
+      "mailto:hello@mintel.me?subject=Q%26A%3A%2050%25%3F&body=a%3Db%20%231",
+    );
+  });
+
+  it("keeps umlauts readable for the mail app", () => {
+    expect(mailtoHref(parts, { subject: "Grüße", body: "ä" })).toBe(
+      "mailto:hello@mintel.me?subject=Gr%C3%BC%C3%9Fe&body=%C3%A4",
+    );
+  });
+
+  it("leaves out an empty subject or body instead of sending empty fields", () => {
+    expect(mailtoHref(parts, { subject: "", body: "" })).toBe(
+      "mailto:hello@mintel.me",
+    );
+    expect(mailtoHref(parts, { subject: "Only subject", body: "" })).toBe(
+      "mailto:hello@mintel.me?subject=Only%20subject",
+    );
+    expect(mailtoHref(parts, { subject: "", body: "Only body" })).toBe(
+      "mailto:hello@mintel.me?body=Only%20body",
+    );
+  });
+});