fix(content_management): add missing import for MaterialApp

- Added 'package:flutter/material.dart' import to create_source_bloc.dart
- This import is necessary for using MaterialApp in the app's UI

Author: fulleni

lib/content_management/bloc/create_source/create_source_bloc.dart

@@ -3,6 +3,7 @@ import 'package:core/core.dart';
 import 'package:country_picker/country_picker.dart' as picker;
 import 'package:data_repository/data_repository.dart';
 import 'package:equatable/equatable.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_news_app_web_dashboard_full_source_code/shared/shared.dart';
 import 'package:language_picker/languages.dart';
 import 'package:uuid/uuid.dart';