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-lock.json

@@ -51,6 +51,7 @@
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         "is-hotkey": "0.2.0",
         "jotai": "2.20.0",
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         "linkify-react": "4.3.3",
         "linkifyjs": "4.3.3",
         "matrix-js-sdk": "41.6.0-rc.0",
@@ -83,6 +84,7 @@
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         "@types/file-saver": "2.0.7",
         "@types/is-hotkey": "0.1.10",
+        "@types/katex": "0.16.8",
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@@ -9087,6 +9096,31 @@
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+    "node_modules/katex": {
+      "version": "0.16.11",
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+        "https://github.com/sponsors/katex"
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+      "license": "MIT",
+      "dependencies": {
+        "commander": "^8.3.0"
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+      "bin": {
+        "katex": "cli.js"
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+      "engines": {
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