refactor: remove flutter_highlight dependency and enhance code block styling

- Removed flutter_highlight as a dependency to streamline the codebase.
- Updated CodeBlockHeader to improve styling based on the current theme, enhancing visual consistency.
- Adjusted code block background and text colors to match GitHub/Atom themes for better readability.
- Refactored markdown processing logic to eliminate reliance on deprecated highlight features, improving maintainability.

pubspec.yaml

@@ -31,7 +31,6 @@ dependencies:
   
   # UI Components - Markdown Rendering
   cached_network_image: ^3.3.1
-  flutter_highlight: ^0.7.0
   flutter_markdown_plus: ^1.0.5
   markdown: ^7.3.0
   webview_flutter: ^4.7.0