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.

lib/shared/widgets/markdown/markdown_preprocessor.dart

@@ -1,5 +1,5 @@
 /// Utility helpers for normalising markdown content before handing it to
-/// [GptMarkdown]. The goal is to keep streaming responsive while smoothing
+/// [ConduitMarkdown]. The goal is to keep streaming responsive while smoothing
 /// out troublesome edge-cases (e.g. nested fences inside lists).
 class ConduitMarkdownPreprocessor {
   const ConduitMarkdownPreprocessor._();