feat(localization): Implement Swedish localization and error messages

- Added localization support for Swedish and English languages.
- Integrated localized strings for user messages in the API error mapper.
- Updated UI components to use localized strings for labels and messages.
- Ensured all error messages are context-aware and utilize the localization framework.
- Created regression test to prevent common ASCII fallbacks in Swedish UI text.

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

@@ -19,7 +19,7 @@ class AuthRepository {
       if (data is! Map<String, dynamic>) {
         throw const ApiException(
           type: ApiErrorType.unknown,
-          message: 'Ogiltigt svar fran servern.',
+          message: 'Ogiltigt svar från servern.',
         );
       }
 
@@ -38,7 +38,7 @@ class AuthRepository {
     } catch (_) {
       throw const ApiException(
         type: ApiErrorType.network,
-        message: 'Kunde inte na servern.',
+        message: 'Kunde inte nå servern.',
       );
     }
   }

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

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../../../core/api/api_error_mapper.dart';
+import '../../../core/l10n/l10n.dart';
 import '../data/auth_providers.dart';
 
 class LoginScreen extends ConsumerStatefulWidget {
@@ -41,9 +42,10 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
   Widget build(BuildContext context) {
     final authState = ref.watch(authStateProvider);
     final isLoading = authState is AsyncLoading;
+    final l10n = context.l10n;
 
     return Scaffold(
-      appBar: AppBar(title: const Text('Logga in')),
+      appBar: AppBar(title: Text(l10n.loginTitle)),
       body: Center(
         child: SingleChildScrollView(
           padding: const EdgeInsets.all(24),
@@ -57,8 +59,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                 children: [
                   TextFormField(
                     controller: _usernameCtrl,
-                    decoration:
-                        const InputDecoration(labelText: 'Användarnamn'),
+                    decoration: InputDecoration(labelText: l10n.usernameLabel),
                     textInputAction: TextInputAction.next,
                     autofocus: true,
                     enabled: !isLoading,
@@ -66,7 +67,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                         FocusScope.of(context).requestFocus(_passwordFocus),
                     validator: (value) {
                       if (value == null || value.trim().isEmpty) {
-                        return 'Ange ditt användarnamn.';
+                        return l10n.usernameRequired;
                       }
                       return null;
                     },
@@ -75,14 +76,14 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                   TextFormField(
                     controller: _passwordCtrl,
                     focusNode: _passwordFocus,
-                    decoration: const InputDecoration(labelText: 'Lösenord'),
+                    decoration: InputDecoration(labelText: l10n.passwordLabel),
                     obscureText: true,
                     textInputAction: TextInputAction.done,
                     enabled: !isLoading,
                     onFieldSubmitted: (_) => _submit(),
                     validator: (value) {
                       if (value == null || value.isEmpty) {
-                        return 'Ange ditt lösenord.';
+                        return l10n.passwordRequired;
                       }
                       return null;
                     },
@@ -93,13 +94,13 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                   else
                     FilledButton(
                       onPressed: _submit,
-                      child: const Text('Logga in'),
+                      child: Text(l10n.loginAction),
                     ),
                   if (authState is AsyncError)
                     Padding(
                       padding: const EdgeInsets.only(top: 16),
                       child: Text(
-                        mapErrorToUserMessage(authState.error!),
+                        mapErrorToUserMessage(authState.error!, context),
                         textAlign: TextAlign.center,
                         style: TextStyle(
                             color: Theme.of(context).colorScheme.error),