feat: implement API client with JSON handling and error mapping; enhance routing and state management in app shell

flutter/lib/features/auth/presentation/login_screen.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+
+import '../../../core/api/api_error_mapper.dart';
 import '../data/auth_providers.dart';
 
 class LoginScreen extends ConsumerStatefulWidget {
@@ -72,7 +74,8 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
               Padding(
                 padding: const EdgeInsets.only(top: 12),
                 child: Text(
-                  'Inloggning misslyckades',
+                  mapErrorToUserMessage(authState.error!),
+                  textAlign: TextAlign.center,
                   style: TextStyle(color: Theme.of(context).colorScheme.error),
                 ),
               ),