fix: update login functionality to use username instead of email

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

@@ -11,19 +11,19 @@ class LoginScreen extends ConsumerStatefulWidget {
 }
 
 class _LoginScreenState extends ConsumerState<LoginScreen> {
-  final _emailCtrl = TextEditingController();
+  final _usernameCtrl = TextEditingController();
   final _passwordCtrl = TextEditingController();
 
   @override
   void dispose() {
-    _emailCtrl.dispose();
+    _usernameCtrl.dispose();
     _passwordCtrl.dispose();
     super.dispose();
   }
 
   Future<void> _submit() async {
     await ref.read(authStateProvider.notifier).login(
-          _emailCtrl.text.trim(),
+          _usernameCtrl.text.trim(),
           _passwordCtrl.text,
         );
     if (mounted) {
@@ -45,9 +45,8 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
           mainAxisAlignment: MainAxisAlignment.center,
           children: [
             TextField(
-              controller: _emailCtrl,
-              decoration: const InputDecoration(labelText: 'E-post'),
-              keyboardType: TextInputType.emailAddress,
+              controller: _usernameCtrl,
+              decoration: const InputDecoration(labelText: 'Anvandarnamn'),
             ),
             const SizedBox(height: 12),
             TextField(