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>

package.json

@@ -76,6 +76,7 @@
     "immer": "11.1.8",
     "is-hotkey": "0.2.0",
     "jotai": "2.20.0",
+    "katex": "0.16.11",
     "linkify-react": "4.3.3",
     "linkifyjs": "4.3.3",
     "matrix-js-sdk": "41.6.0-rc.0",
@@ -108,6 +109,7 @@
     "@types/chroma-js": "3.1.2",
     "@types/file-saver": "2.0.7",
     "@types/is-hotkey": "0.1.10",
+    "@types/katex": "0.16.8",
     "@types/node": "25.9.1",
     "@types/prismjs": "1.26.6",
     "@types/react": "19.2.15",