style(dart): organize imports for consistency

- Reorder imports to follow Dart guidelines
- Add missing 'package:flutter/foundation.dart' import

Author: fulleni

lib/app/config/app_config.dart

@@ -1,6 +1,7 @@
-import 'package:flutter_news_app_mobile_client_full_source_code/app/config/app_environment.dart';
 import 'dart:io' show Platform;
 
+import 'package:flutter_news_app_mobile_client_full_source_code/app/config/app_environment.dart';
+
 /// {@template app_config}
 /// A centralized configuration class that provides all necessary
 /// environment-specific variables for the application.