feat(web): add LaTeX math formula rendering with KaTeX (#436)

* feat(web): add LaTeX math formula rendering with KaTeX

Add remark-math + rehype-katex to the markdown rendering pipeline
so inline ($...$) and display ($$...$$) math formulas are rendered
as proper KaTeX output in chat messages and tool results.

Closes #237

* fix(web): disable single-dollar math parsing and add KaTeX to reasoning

- Set singleDollarTextMath: false to prevent $HOME, $PATH etc from
  being misinterpreted as math formulas. Only $$...$$ (display) is
  parsed; inline math requires explicit \(...\) or $$...$$.
- Add rehypePlugins to the reasoning renderer so math formulas
  render consistently across chat, tool results, and reasoning blocks.

* refactor(web): use satisfies for type-safe plugin exports

Replace any[] with satisfies NonNullable<MarkdownTextPrimitiveProps[...]>
to preserve type safety on the shared plugin lists without needing
eslint suppressions.

* fix(web): enable single-dollar inline math syntax

Re-enable $...$ parsing (remark-math default) so inline formulas
like $E=mc^2$ render correctly. Shell variables like $HOME typically
appear inside code spans/blocks which remark-math does not parse,
so false positives are minimal in practice.

web/src/index.css

@@ -1,4 +1,5 @@
 @import "tailwindcss";
+@import "katex/dist/katex.min.css";
 @config "../tailwind.config.ts";
 
 :root {