fix(migration): reset file input value on cancel

resetToUpload() cleared state but not the file input's value, so
re-selecting the same file after cancel would not trigger a change
event in the browser.

frontend/src/views/migrate/MigrationCSV.vue

@@ -363,6 +363,9 @@ function resetToUpload() {
 	detectionResult.value = null
 	previewResult.value = null
 	error.value = ''
+	if (uploadInput.value) {
+		uploadInput.value.value = ''
+	}
 	config.value = {
 		delimiter: ',',
 		quote_char: '"',