#2974 web: render chat math with bundled KaTeX

Description

Depends on #2972. Minimal replacement for #1830.

Enable Render math in Chat settings to render inline and display equations with locally bundled KaTeX 0.19.0. Preserves original source; invalid input falls back to source. Disabled by default; no CDN.

Renderer changes contain only the adapter and required assets, license and provenance. Validated in Chromium, Firefox and Playwright WebKit.