replaced account data singleton with a session model

lib/utils/downloads/download_manager.dart

@@ -6,8 +6,8 @@ import 'package:background_downloader/background_downloader.dart' as bd;
 import 'package:flutter/foundation.dart';
 
 import '../../api/marianumcloud/webdav/webdav_api.dart';
-import '../../model/account_data.dart';
 import '../../notification/notification_service.dart';
+import '../../session/session_manager.dart';
 import '../../share_intent/remote_file_ref.dart';
 import 'download_job.dart';
 
@@ -116,7 +116,7 @@ class DownloadManager {
     final encodedPath = Uri.encodeComponent(remotePath).replaceAll('%2F', '/');
     final task = bd.DownloadTask(
       url: '${WebdavApi.buildWebdavUrl()}$encodedPath',
-      headers: AccountData().authHeaders(),
+      headers: SessionManager().requireNextcloud().authHeaders,
       filename: name,
       baseDirectory: bd.BaseDirectory.temporary,
       directory: _directory,