feat(mail): one layout for all mails with logo, greeting, cards and footer

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

internal/api/mail.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"crypto/rand"
 	"crypto/tls"
+	"encoding/base64"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -147,7 +148,27 @@ func buildMessage(from, to, subject, body, html string, now time.Time) []byte {
 	fmt.Fprintf(&buf, "--%s\r\n", boundary)
 	part("text/plain", body)
 	fmt.Fprintf(&buf, "--%s\r\n", boundary)
-	part("text/html", html)
+	if strings.Contains(html, "cid:"+mailLogoCID) {
+		// The HTML shows the logo from inside the mail: HTML and image together as multipart/related
+		related := boundary + "-rel"
+		fmt.Fprintf(&buf, "Content-Type: multipart/related; boundary=\"%s\"\r\n\r\n", related)
+		fmt.Fprintf(&buf, "--%s\r\n", related)
+		part("text/html", html)
+		fmt.Fprintf(&buf, "--%s\r\n", related)
+		buf.WriteString("Content-Type: image/png; name=\"mindsift.png\"\r\n")
+		buf.WriteString("Content-Transfer-Encoding: base64\r\n")
+		fmt.Fprintf(&buf, "Content-ID: <%s>\r\n", mailLogoCID)
+		buf.WriteString("Content-Disposition: inline; filename=\"mindsift.png\"\r\n\r\n")
+		enc := base64.StdEncoding.EncodeToString(mailLogoPNG)
+		for len(enc) > 76 {
+			buf.WriteString(enc[:76] + "\r\n")
+			enc = enc[76:]
+		}
+		buf.WriteString(enc + "\r\n")
+		fmt.Fprintf(&buf, "--%s--\r\n", related)
+	} else {
+		part("text/html", html)
+	}
 	fmt.Fprintf(&buf, "--%s--\r\n", boundary)
 	return buf.Bytes()
 }