MarkdownWidget: Add syntax highlighting for fenced code blocks

[mitchdenny]

Summary

Fenced code blocks in the MarkdownWidget currently render as read-only EditorWidget instances with line numbers but no syntax highlighting. The language info string (e.g. ```csharp) is parsed and available via FencedCodeBlock.Language` but not yet used for colorization.

Proposed Approach

Integrate with the Editor widget's language server / tokenization infrastructure (see #165) to provide syntax-aware highlighting. This would involve:

1. Mapping the markdown language tag to an editor language mode
2. Applying token-based styling to the read-only editor content
3. Falling back to plain text when no language mode is available

Dependencies

• Depends on Editor widget extensible language/tokenization support (feat: Editor Widget — extensible document editing with piece tree, multi-cursor, and collaboration-ready architecture #165)

Current State

• FencedCodeBlock.Language is already parsed and stored
• Code blocks render via MarkdownCodeBlockWidget wrapping EditorWidget in read-only mode
• Line numbers are shown; scroll bindings are removed so parent scroll panels work correctly

Labels

Enhancement, MarkdownWidget