add .GLD file verification for drag and drop

src/routes/(token-based)/list/[vorgang]/+page.svelte

@@ -293,13 +293,21 @@ Mit freundlichen Grüßen,
 	// drag and drop functionality
 		let isDragging = $state(false);
 
-		function handleDrop(event) {
+		async function handleDrop(event) {
 			event.preventDefault();
 			isDragging = false;
 
 			if (event.dataTransfer?.files?.length) {
 				files = event.dataTransfer.files;
 			}
+			if (!(await check_valid_glb_file())) {
+				formErrors = { file: 'Keine gültige .GLD-Datei' }
+				// reset form fields etc.
+				files = null;
+				fileInput.value = '';
+			} else {
+				formErrors = { ...formErrors, file: ''}
+			};
 		}
 
 </script>