GitHub Better LaTeX

Re-render LaTeX in GitHub Markdown.

Render LaTeX in GitHub Markdown and repair broken math in Copilot Chat. - Re-renders repository Markdown blob pages ending in `.md`, using the original Markdown source and KaTeX. - Repairs failed display-math blocks in Copilot Chat when GitHub's error fallback still contains the original TeX source. - Re-renders Copilot message bodies from original Markdown when it is available in the message's React props. Markdown math supports `$...$` and `\(...\)` for inline expressions, and `$$...$$` and `\[...\]` for display expressions. Copilot message controls are preserved; GitHub-specific body widgets appear as standard Markdown output.