feat(messages): KaTeX math rendering (P4-4)

Renders LaTeX via spec data-mx-maths spans/divs (KaTeX render of the attr,
children as fallback) and conservative $…$ / $$…$$ text detection (escape-aware,
currency-guarded, never inside code/pre). KaTeX + CSS load lazily on first math
(ReactPrism pattern) — verified absent from the eager bundle. Sanitizer
unchanged by design (we render post-sanitize from attr/text; no incoming MathML
accepted). +14 unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/math/KaTeX.tsx

@@ -0,0 +1,41 @@
+import React from 'react';
+import katex from 'katex';
+import 'katex/dist/katex.min.css';
+
+type KaTeXProps = {
+  /** Raw LaTeX source (without `$`/`$$` delimiters). */
+  latex: string;
+  /** Render as block (display) math when true, inline otherwise. */
+  displayMode?: boolean;
+};
+
+/**
+ * Lazily-loaded KaTeX renderer.
+ *
+ * This module statically imports `katex` and its stylesheet, so both only enter
+ * the bundle via the dynamic `import()` of this file (see the `lazy()` wrapper
+ * in `react-custom-html-parser.tsx`). They are therefore NOT part of the eager
+ * import graph.
+ *
+ * We render with `throwOnError: false`, so KaTeX itself renders a parse error
+ * inline (in its error colour) rather than throwing. The HTML returned by
+ * `renderToString` is produced by our own trusted call from a fixed options
+ * object — it is safe to inject via `dangerouslySetInnerHTML`.
+ */
+export default function KaTeX({ latex, displayMode = false }: KaTeXProps) {
+  const html = katex.renderToString(latex, {
+    displayMode,
+    throwOnError: false,
+    output: 'htmlAndMathml',
+  });
+
+  const Wrapper = displayMode ? 'div' : 'span';
+
+  return (
+    <Wrapper
+      // KaTeX output is generated by our own render call (trusted-safe).
+      // eslint-disable-next-line react/no-danger
+      dangerouslySetInnerHTML={{ __html: html }}
+    />
+  );
+}