#2975 web: render chat math with bundled MathJax

本文

Depends on #2972. Minimal replacement for #1832; MathJax alternative to #2974.

Enable Render math in Chat settings to render inline and display equations with locally bundled MathJax 4.1.3 and New Computer Modern fonts. Preserves original source; invalid input falls back to source. Disabled by default; no CDN.

Contains only the adapter and required assets, licenses and provenance. Validated in Chromium, Firefox and Playwright WebKit.