added support for multiple accounts, guardian login bugfixes, ui changes

lib/api/marianumcloud/app_password/delete_app_password.dart

@@ -16,10 +16,15 @@ class DeleteAppPassword {
   Future<void> run({String? authorizationHeader}) async {
     await _client.delete(
       NextcloudOcs.uri('core/apppassword'),
-      headers: {
-        ...NextcloudOcs.headers(),
-        'Authorization': ?authorizationHeader,
-      },
+      // An explicit header may belong to an inactive account, so the active
+      // session's headers must not be required then.
+      headers: authorizationHeader == null
+          ? NextcloudOcs.headers()
+          : {
+              'Accept': 'application/json',
+              'OCS-APIRequest': 'true',
+              'Authorization': authorizationHeader,
+            },
     );
   }
 }

lib/api/marianumcloud/webdav/webdav_api.dart

@@ -18,7 +18,8 @@ abstract class WebdavApi<T> {
   /// changes (app password minted/renewed, account switch) so it never keeps
   /// authenticating with stale credentials.
   static Future<WebDavClient> get webdav {
-    final secret = SessionManager().requireNextcloud().secret;
+    final nextcloud = SessionManager().requireNextcloud();
+    final secret = '${nextcloud.username}:${nextcloud.secret}';
     if (_webdav == null || _webdavSecret != secret) {
       _webdavSecret = secret;
       _webdav = establishWebdavConnection();