fix & optimize

- fix downloading page unbond size
- auto dispose isolate

lib/data/model/sftp/download_status.dart

@@ -33,6 +33,9 @@ class SftpDownloadStatus {
     switch (event.runtimeType) {
       case SftpWorkerStatus:
         status = event;
+        if (status == SftpWorkerStatus.finished) {
+          worker.dispose();
+        }
         break;
       case double:
         progress = event;

lib/data/model/sftp/download_worker.dart

@@ -24,6 +24,10 @@ class SftpDownloadWorker {
   final worker = Worker();
   final String? privateKey;
 
+  void dispose() {
+    worker.dispose();
+  }
+
   /// Initiate the worker (new thread) and start listen from messages between
   /// the threads
   Future<void> init() async {