WebUI: always provide event variable

This is unifying coding style and avoid wrong usages.

PR #22676.

src/webui/www/private/confirmfeeddeletion.html

@@ -12,7 +12,7 @@
     <script>
         "use strict";
 
-        window.addEventListener("DOMContentLoaded", () => {
+        window.addEventListener("DOMContentLoaded", (event) => {
             document.getElementById("cancelBtn").focus();
             document.getElementById("cancelBtn").addEventListener("click", (e) => {
                 e.preventDefault();