Add early return guard for undefined uploadFile before accessing .size

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

src/components/File/UploadModal.vue

@@ -69,6 +69,7 @@ async function upload(event) {
   if (target && target.files) {
     uploadFile = target.files[0];
   }
+  if (!uploadFile) return;
   // File size must not exceed 90 MB (90*1024*1024 = 94,371,840 bytes)
   if (uploadFile.size >= 94371840) {
     fileInput.value = "";