feat(desktop): render LaTeX math via KaTeX after streaming completes

Add @streamdown/math plugin to the chat markdown renderer.
Inline ($x^2$) and block ($$...$$) math both supported with
singleDollarTextMath enabled. Plugin is gated to non-streaming state
to match the existing pattern for syntax highlighting — math renders
when the message completes, avoiding KaTeX re-render churn during
streaming. KaTeX CSS is imported in styles.css; ~30KB CSS + ~430KB
JS added to the bundle. Smoothness improvements during streaming
deferred to a follow-up.

apps/desktop/src/styles.css

@@ -1,6 +1,7 @@
 @import 'tailwindcss';
 @plugin '@tailwindcss/typography';
 @import 'tw-shimmer';
+@import 'katex/dist/katex.min.css';
 @custom-variant dark (&:is(.dark *));
 
 @font-face {