Rollback Tailwind CSS from v4 to v3.4.18

Tailwind CSS 4 has significant breaking changes that broke the UI.
Rolled back to stable v3.4.18 while keeping other package updates.

Changes:
- tailwindcss: 4.1.17 → 3.4.18
- Removed @tailwindcss/postcss (not needed for v3)
- Reverted postcss.config.js to use 'tailwindcss' plugin

All other package updates remain (React 19, Vite 7, etc.)

Author: Rafael Teodoro