chore: update dependencies and refactor Tailwind CSS integration

- Upgraded @vitejs/plugin-react to version 5
- Upgraded tailwindcss to version 4 and added @tailwindcss/vite
- Removed postcss.config.js and tailwind.config.js files
- Updated index.css to use @import for Tailwind CSS
- Modified vite.config.ts to include tailwindcss plugin

Author: sheepsteak