refactor: KaTeX integration with SCSS

Moves KaTeX styling to SCSS for better customization and font embedding.

This change includes:
- Adding `sass` as a dev dependency.
- Introducing a custom SCSS file to override KaTeX variables and disable TTF/WOFF fonts, relying solely on WOFF2 for embedding.
- Adjusting the Vite configuration to resolve `katex-fonts` alias and inject SCSS variables.

Author: allozaur