docs(pubspec): update package description

- Replace generic description with a more detailed explanation
- Highlight key features: authentication, data management, user settings
- Mention Dart Frog as the technology used for implementation

Author: fulleni

pubspec.yaml

@@ -1,5 +1,5 @@
 name: flutter_news_app_api_server_full_source_code
-description: ...
+description: The complete backend API server for the Flutter News App Toolkit, built with Dart Frog. Powers authentication, data management, user settings, and more.
 repository: https://github.com/flutter-news-app-full-source-code/flutter-news-app-api-server-full-source-code
 publish_to: none