Hide drop zone container during upload to fix empty border flash

The outer bordered div was always in the DOM; only the label and input
inside were hidden during upload, leaving a visible empty dashed box.
Apply the same hidden guard to the container div.

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

src/routes/events/ae_comp__event_files_upload.svelte

@@ -219,7 +219,8 @@ async function handle_input_upload_files({
             cursor-pointer rounded-lg border-2
             border-dashed p-1
             transition-colors hover:border-2
-            ">
+            "
+        class:hidden={$events_sess.files.disable_submit__event_file_obj}>
         <label
             for={input_element_id}
             class="