feat(demo): redesign demo site with MUI-inspired layout

- Add professional Header with nav, GitHub/NPM links
- Add Hero section with tagline, install command, feature pills
- Add CodeBlock component with collapsible source code
- Add DemoSection component for consistent demo layout
- Add Footer with links and resources
- Create 4 interactive demos: Basic, Theming, Hot Zones, Multi-Band
- Add Installation/Quick Start section
- Add API reference table
- Add CTA section with gradient background
- Update E2E tests for new demo structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: thbst16