[WIP] Add live LaTeX / math preview support in editor

[Codex]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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This section details on the original issue you should resolve

<issue_title>Feature Request: Live LaTeX / Math Preview</issue_title>
<issue_description>## Feature Request: Live LaTeX / Math Preview

Problem

GitHub natively supports LaTeX math rendering in comments using $...$ (inline) and $$...$$ (block) syntax via MathJax. However, the extension currently has no preview support for LaTeX. Math expressions are shown as raw text while editing, breaking the WYSIWYG experience.

Expected Behavior

Math expressions should render live in the editor, matching what GitHub will display after submission/in preview:

• Inline: $E = mc^2$ → $E = mc^2$ (rendered inline math)
• Block:

$$
\int_0^\infty e^{-x^2} dx = \frac{\sqrt{\pi}}{2}
$$

→
$$\int_0^\infty e^{-x^2} dx = \frac{\sqrt{\pi}}{2}$$
(rendered as a math block in a new line)

Suggested Implementation

Since the editor is already built on ProseMirror, prosemirror-math with KaTeX could be a good fit — it provides inline and block math node support with live rendering and is well-maintained.

Context

This would be especially useful for users writing math-heavy issues, discussions, or PR reviews (e.g. in scientific/ML/engineering repos).</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

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[riccardoperra]

@codex[agent] consider to use https://prosekit.dev/extensions/math/ which already defines node and inline blocks. The extension should be added in the general markdown schema with his own test files, matching all other nodes. Also use this as a reference: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions

[riccardoperra]

@codex[agent] consider to use https://prosekit.dev/extensions/math/ which already defines node and inline blocks. The extension should be added in the general markdown schema with his own test files, matching all other nodes. Also use this as a reference: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions