simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/queries/auth_login/auth_login.dart

@@ -1,6 +1,7 @@
 import 'package:dio/dio.dart';
 
 import '../../auth/token_storage.dart';
+import '../../marianumconnect_api.dart';
 import '../../marianumconnect_query.dart';
 import 'auth_login_response.dart';
 
@@ -9,9 +10,6 @@ import 'auth_login_response.dart';
 /// run through the shared dio instance — that one has the interceptor, which
 /// would attempt to re-auth us into a loop if our credentials are wrong.
 class AuthLogin extends MarianumConnectQuery {
-  static const Duration _connectTimeout = Duration(seconds: 10);
-  static const Duration _receiveTimeout = Duration(seconds: 15);
-
   final MarianumConnectTokenStorage _tokenStorage;
 
   AuthLogin({
@@ -19,17 +17,7 @@ class AuthLogin extends MarianumConnectQuery {
         const MarianumConnectTokenStorage(),
     Dio? dio,
   }) : _tokenStorage = tokenStorage,
-       super(dio: dio ?? _buildDio());
-
-  static Dio _buildDio() => Dio(
-    BaseOptions(
-      connectTimeout: _connectTimeout,
-      receiveTimeout: _receiveTimeout,
-      sendTimeout: _connectTimeout,
-      responseType: ResponseType.json,
-      contentType: 'application/json',
-    ),
-  );
+       super(dio: dio ?? MarianumConnectApi.plainDio());
 
   Future<AuthLoginResponse> run({
     required String username,