simplify: dedupe boilerplate and remove dead code across all layers

lib/api/marianumconnect/queries/auth_verify/auth_verify.dart

@@ -2,6 +2,7 @@ import 'package:dio/dio.dart';
 
 import '../../../errors/auth_exception.dart';
 import '../../auth/token_storage.dart';
+import '../../marianumconnect_api.dart';
 import '../../marianumconnect_query.dart';
 
 /// Probes that the stored bearer token still maps to the given credentials.
@@ -12,9 +13,6 @@ import '../../marianumconnect_query.dart';
 /// Bypasses the shared dio singleton so the auth interceptor doesn't kick in
 /// and obscure a real 401 with a silent re-login.
 class AuthVerify extends MarianumConnectQuery {
-  static const Duration _connectTimeout = Duration(seconds: 10);
-  static const Duration _receiveTimeout = Duration(seconds: 15);
-
   final MarianumConnectTokenStorage _tokenStorage;
 
   AuthVerify({
@@ -22,17 +20,7 @@ class AuthVerify extends MarianumConnectQuery {
         const MarianumConnectTokenStorage(),
     Dio? dio,
   }) : _tokenStorage = tokenStorage,
-       super(dio: dio ?? _buildDio());
-
-  static Dio _buildDio() => Dio(
-    BaseOptions(
-      connectTimeout: _connectTimeout,
-      sendTimeout: _connectTimeout,
-      receiveTimeout: _receiveTimeout,
-      responseType: ResponseType.json,
-      contentType: 'application/json',
-    ),
-  );
+       super(dio: dio ?? MarianumConnectApi.plainDio());
 
   /// Throws [AuthException] on 401 (credentials no longer match the token's
   /// user, token missing, or token rejected), other [AppException]s on