feat: Latex support (#929)

* feat: implement LaTeX math rendering via KaTeX

- Add katex, remark-math, and rehype-katex dependencies
- Integrate KaTeX CSS with light/dark theme color overrides
- Wire remark-math and rehype-katex plugins into MarkdownRenderer
- Support inline ($...$) and display ($$...$$) math delimiters
- Add plan and task documentation

* fix(katex): display inline error messages for invalid LaTeX

- Configure rehypeKatex with throwOnError: false to render errors inline
- Use destructive color var for error text
- Update feature documentation with verification results

* chore: remove latex PR artifacts and hardcoded color fallback

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Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/package.json

@@ -67,8 +67,11 @@
     "heic2any": "^0.0.4",
     "html-to-image": "^1.11.13",
     "http-proxy-middleware": "^3.0.5",
+    "katex": "^0.16.21",
     "marked": "^17.0.3",
     "morphdom": "^2.7.7",
+    "rehype-katex": "^7.0.1",
+    "remark-math": "^6.0.0",
     "motion": "^12.23.24",
     "next-themes": "^0.4.6",
     "prismjs": "^1.30.0",