fix & opt.

lib/data/model/sftp/download_item.dart

@@ -0,0 +1,16 @@
+import '../server/server_private_info.dart';
+
+class DownloadItem {
+  DownloadItem(this.spi, this.remotePath, this.localPath);
+
+  final ServerPrivateInfo spi;
+  final String remotePath;
+  final String localPath;
+}
+
+class DownloadItemEvent {
+  DownloadItemEvent(this.item, this.privateKey);
+
+  final DownloadItem item;
+  final String? privateKey;
+}