Add optional chaining for querySelector result in Files.vue

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/views/Files/Files.vue

@@ -630,7 +630,7 @@
     });
     // 為 DataTable tbody 加入 .scrollbar 選擇器
     const tbodyElement = document.querySelector('.p-datatable-tbody');
-    tbodyElement.classList.add('scrollbar');
+    tbodyElement?.classList.add('scrollbar');
     isLoading.value = false;
   });
 </script>