data exporting & importing

lib/foundation/history.dart

@@ -172,6 +172,8 @@ class HistoryManager with ChangeNotifier {
           max_page int
         );
       """);
+
+    notifyListeners();
   }
 
   /// add history. if exists, update time.
@@ -275,4 +277,8 @@ class HistoryManager with ChangeNotifier {
     """);
     return res.first[0] as int;
   }
+
+  void close() {
+    _db.dispose();
+  }
 }