fix: Add PostCSS config for Tailwind v4 and fix CSS build

- Added postcss.config.js with @tailwindcss/postcss plugin
- Installed @tailwindcss/postcss package
- Removed incompatible @apply directives from index.css
- Fixed broken UI where Tailwind CSS was not compiling (CSS bundle was <1KB, now 6.21KB)
- Resolves unstyled interface in GitHub Codespaces deployment

Author: TamTunnel