fix(CouponCodeInput): mobile buttons adjusted

Related: quiqqer/template-cologne#103

src/QUI/ERP/Coupons/CodeGenerators/SimpleString.php

@@ -30,7 +30,7 @@ class SimpleString implements CodeGeneratorInterface
      * @return string
      * @throws Exception
      */
-    public static function generate($prefix = ''): string
+    public static function generate(string $prefix = ''): string
     {
         $characters = array_merge(
             range('A', 'Z'),