Security improvements:
- Add rehype-sanitize for XSS protection in markdown content
- Remove DOMPurify and deprecated __html code block feature
- Extract sanitize schema to constants with comprehensive documentation

Maintainability improvements:
- Extract SANITIZE_SCHEMA to constants.ts for better organization
- Create utils.ts with shared code extraction utilities
- Refactor CodeBlock and MermaidBlock to use shared utilities
- Rename PreProps to CodeBlockProps for clarity
- Reduce code duplication across components

Dependency cleanup:
- Remove explicit katex dependency (now transitive via rehype-katex)
- Remove @matejmazur/react-katex (unused)
- Remove dompurify (replaced by rehype-sanitize)
- Update vite config to remove katex-vendor chunk

Changes: 7 files changed, 84 insertions(+), 100 deletions(-)

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>