fix: update login functionality to use username instead of email

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

@@ -8,16 +8,16 @@ class AuthRepository {
 
   AuthRepository(this._api, this._storage);
 
-  Future<String> login(String email, String password) async {
+  Future<String> login(String username, String password) async {
     final response = await _api.post(
       '/auth/login',
-      jsonEncode({'email': email, 'password': password}),
+      jsonEncode({'username': username, 'password': password}),
     );
     if (response.statusCode != 200 && response.statusCode != 201) {
       throw Exception('Login failed: ${response.statusCode}');
     }
     final data = jsonDecode(response.body) as Map<String, dynamic>;
-    final token = data['access_token'] as String;
+    final token = data['accessToken'] as String;
     await _storage.saveToken(token);
     return token;
   }