Add animated particles background effect

- Create ParticlesBackground component with SCSS-generated particles
- Add floating particle animation across all pages
- Light mode: subtle indigo particles
- Dark mode: white particles
- Particles appear behind all content with proper z-indexing

Based on: https://codepen.io/alexitaylor/pen/RgxJwg

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: clucraft