added support for 2fa login with browser flow

lib/api/marianumcloud/webdav/webdav_api.dart

@@ -7,18 +7,31 @@ import '../../api_response.dart';
 abstract class WebdavApi<T> {
   T genericParams;
 
-  WebdavApi(this.genericParams) {
-    establishWebdavConnection();
-  }
+  WebdavApi(this.genericParams);
 
   Future<ApiResponse> run();
 
-  static Future<WebDavClient> webdav = establishWebdavConnection();
+  static Future<WebDavClient>? _webdav;
+  static String? _webdavSecret;
+
+  /// Shared WebDAV client. Rebuilt whenever the effective Nextcloud secret
+  /// changes (app password minted/renewed, account switch) so it never keeps
+  /// authenticating with stale credentials.
+  static Future<WebDavClient> get webdav {
+    final secret = AccountData().getNextcloudSecret();
+    if (_webdav == null || _webdavSecret != secret) {
+      _webdavSecret = secret;
+      _webdav = establishWebdavConnection();
+    }
+    return _webdav!;
+  }
 
   static Future<WebDavClient> establishWebdavConnection() async =>
       NextcloudClient(
         Uri.parse('https://${EndpointData().nextcloud().full()}'),
-        password: AccountData().getPassword(),
+        // App password preferred — with 2FA the real password is not accepted
+        // by Nextcloud at all (see AccountData.usesLoginFlow).
+        password: AccountData().getNextcloudSecret(),
         loginName: AccountData().getUsername(),
       ).webdav;