refactor: use const instead of var in CouponCodeInput.js

Modified 'var' declarations to 'const' in CouponCodeInput.js file for better ES6 compliance. This
refactoring helps increase code readability and ensures variables are block-scoped, reducing the
risk of accidental variable reassignment or mutation.

bin/frontend/controls/CouponCodeInput.js

@@ -23,7 +23,7 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
 ], function(QUI, QUIControl, QUILoader, Sessions, CouponCodes, QUIAjax, QUILocale, Mustache, template) {
     'use strict';
 
-    var lg = 'quiqqer/coupons';
+    const lg = 'quiqqer/coupons';
 
     return new Class({
 
@@ -50,8 +50,8 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
          * Event: onInject
          */
         $onInject: function() {
-            var self = this;
-            var lgPrefix = 'controls.frontend.CouponCodeInput.template.';
+            const self = this;
+            const lgPrefix = 'controls.frontend.CouponCodeInput.template.';
 
             this.$Elm.addClass('quiqqer-coupons-field');
 
@@ -81,7 +81,7 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
             });
 
             if (this.isInOrder()) {
-                var OrderProcess = this.getOrderProcess();
+                const OrderProcess = this.getOrderProcess();
 
                 new Promise(function(resolve) {
                     if (!OrderProcess.isLoaded()) {
@@ -118,8 +118,8 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
                 return;
             }
 
-            var self = this;
-            var code = this.$Input.value.trim();
+            const self = this;
+            const code = this.$Input.value.trim();
 
             if (code === '') {
                 this.$Input.focus();
@@ -192,7 +192,7 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
                 return;
             }
 
-            var OrderProcess = this.getOrderProcess();
+            const OrderProcess = this.getOrderProcess();
 
             OrderProcess.Loader.show();
 
@@ -239,7 +239,7 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
          * @return {boolean}
          */
         isInOrder: function() {
-            var OrderProcessNode = this.getElm().getParent(
+            const OrderProcessNode = this.getElm().getParent(
                 '[data-qui="package/quiqqer/order/bin/frontend/controls/OrderProcess"]'
             );
 
@@ -252,7 +252,7 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
          * @return {Object}
          */
         getOrderProcess: function() {
-            var OrderProcessNode = this.getElm().getParent(
+            const OrderProcessNode = this.getElm().getParent(
                 '[data-qui="package/quiqqer/order/bin/frontend/controls/OrderProcess"]'
             );
 
@@ -269,10 +269,10 @@ define('package/quiqqer/coupons/bin/frontend/controls/CouponCodeInput', [
                 return Promise.resolve();
             }
 
-            var self = this;
+            const self = this;
 
             return new Promise(function(resolve) {
-                var OrderProcess = self.getOrderProcess();
+                const OrderProcess = self.getOrderProcess();
 
                 OrderProcess.Loader.show();
                 OrderProcess.getOrder().then(function(orderHash) {