Show German date format (DD.MM.YYYY) in the event edit form

flatpickr now uses an alternative display input with altFormat d.m.Y,
typed input is parsed in that format as well. The submitted value stays
ISO (Y-m-d), so form handling and validation are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

contao/templates/frontend/event_edit.html.twig

@@ -255,9 +255,12 @@
         if (typeof window.flatpickr === 'function') {
           const dateInputs = document.querySelectorAll('input.js-flatpickr-date');
           dateInputs.forEach(function (input) {
+            // Gespeichert wird weiter ISO (Y-m-d); angezeigt und eingegeben wird deutsch (TT.MM.JJJJ)
             window.flatpickr(input, {
               locale: 'de',
               dateFormat: 'Y-m-d',
+              altInput: true,
+              altFormat: 'd.m.Y',
               allowInput: true
             });
           });