refactor(mail): use CryptoRandomString for Message-ID generation

Replace manual rand.Read + hex.EncodeToString with the existing
utils.CryptoRandomString helper for generating the random part
of the Message-ID header.

pkg/mail/send_mail.go

@@ -17,14 +17,13 @@
 package mail
 
 import (
-	"crypto/rand"
 	"embed"
-	"encoding/hex"
 	"fmt"
 	"io"
 
 	"code.vikunja.io/api/pkg/config"
 	"code.vikunja.io/api/pkg/log"
+	"code.vikunja.io/api/pkg/utils"
 	"code.vikunja.io/api/pkg/version"
 
 	"github.com/wneessen/go-mail"
@@ -83,9 +82,8 @@ func getMessage(opts *Opts) *mail.Msg {
 
 	// Set an RFC 5322 compliant Message-ID using the public URL domain
 	// instead of relying on os.Hostname() which is unreliable in containers.
-	randBytes := make([]byte, 16)
-	_, _ = rand.Read(randBytes)
-	messageID := hex.EncodeToString(randBytes) + "@" + GetMailDomain()
+	randPart, _ := utils.CryptoRandomString(32)
+	messageID := randPart + "@" + GetMailDomain()
 	m.SetMessageIDWithValue(messageID)
 	if opts.From == "" {
 		opts.From = "Vikunja <" + config.MailerFromEmail.GetString() + ">"