chore: update markdown dependency and configuration

- Replaced `gpt_markdown` with `flutter_markdown_plus` in `pubspec.yaml`.
- Updated `pubspec.lock` to reflect the new dependency version.
- Modified `markdown_config.dart` to generalize styling configuration.
- Updated `streaming_markdown_widget.dart` to utilize `MarkdownBody` for rendering markdown content.

pubspec.yaml

@@ -28,7 +28,7 @@ dependencies:
   shared_preferences: ^2.3.2
   
   # UI Components - Enhanced Markdown  
-  gpt_markdown: ^1.1.4
+  flutter_markdown_plus: ^1.0.5
   cached_network_image: ^3.3.1
   socket_io_client: ^3.1.2
   yaml: ^3.1.2