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

flutter/lib/features/auth/data/auth_repository.dart

@@ -1,5 +1,5 @@
-import 'dart:convert';
 import '../../../core/api/api_client.dart';
+import '../../../core/api/api_exception.dart';
 import '../../../core/platform/token_storage.dart';
 
 class AuthRepository {
@@ -9,17 +9,37 @@ class AuthRepository {
   AuthRepository(this._api, this._storage);
 
   Future<String> login(String username, String password) async {
-    final response = await _api.post(
-      '/auth/login',
-      jsonEncode({'username': username, 'password': password}),
-    );
-    if (response.statusCode != 200 && response.statusCode != 201) {
-      throw Exception('Login failed: ${response.statusCode}');
+    try {
+      final data = await _api.postJson(
+        '/auth/login',
+        body: {'username': username, 'password': password},
+      );
+
+      if (data is! Map<String, dynamic>) {
+        throw const ApiException(
+          type: ApiErrorType.unknown,
+          message: 'Ogiltigt svar fran servern.',
+        );
+      }
+
+      final token = data['accessToken'];
+      if (token is! String || token.isEmpty) {
+        throw const ApiException(
+          type: ApiErrorType.unknown,
+          message: 'Svar saknar access token.',
+        );
+      }
+
+      await _storage.saveToken(token);
+      return token;
+    } on ApiException {
+      rethrow;
+    } catch (_) {
+      throw const ApiException(
+        type: ApiErrorType.network,
+        message: 'Kunde inte na servern.',
+      );
     }
-    final data = jsonDecode(response.body) as Map<String, dynamic>;
-    final token = data['accessToken'] as String;
-    await _storage.saveToken(token);
-    return token;
   }
 
   Future<void> logout() => _storage.deleteToken();

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),
                 ),
               ),