fix: Handle undefined File from input element in MigrationView

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

frontend/src/views/admin/MigrationView.vue

@@ -37,7 +37,7 @@ async function createExport() {
 function handleFileSelect(event: Event) {
   const target = event.target as HTMLInputElement
   if (target.files && target.files.length > 0) {
-    uploadFile.value = target.files[0]
+    uploadFile.value = target.files[0] ?? null
   }
 }