don't use BCC for single-recipient email (#395)

Co-authored-by: Copilot <[email protected]>

Author: simonLeary42

resources/lib/UnityMailer.php

@@ -111,7 +111,7 @@ public function sendMail(string $recipients, ?string $template = null, mixed $da
                         $this->addBCC($addr);
                     }
                 } else {
-                    $this->addBCC($recipients);
+                    $this->addAddress($recipients);
                 }
             }
         }