feat(renderer): KaTeX math rendering for $inline$ and $$block$$ (M10.6)

- Lazy-loads KaTeX (~70KB) on first math detected
- Inline $...$ and block $$...$$ LaTeX support
- Falls back to <code> for invalid LaTeX
- Batch render after markdown, not during streaming

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/package.json

@@ -21,6 +21,7 @@
   "dependencies": {
     "@aiui/core": "workspace:*",
     "@tanstack/vue-virtual": "^3.13.19",
+    "katex": "^0.16.33",
     "leaflet": "^1.9.4",
     "markdown-it": "^14.1.1",
     "pdfjs-dist": "^5.5.207",