* 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>
