feat(website): interactive demo cards with animated modal lightbox

- Demo cards: hover lift + scale + image zoom + overlay badge
- Click opens modal with phone mockup, Ken Burns animation on screenshot
- Tab switcher (Explore / Showcase / Effects) with instant content swap
- Features animate in with 55ms stagger delay on open + tab switch
- Keyboard: Esc to close, ← → arrows to cycle tabs
- Responsive: stacked on mobile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: claude