feat: add macOS as third platform in landing page

- Update meta tags, structured data, and social cards to mention macOS
- Add warm orange/amber CSS theme for macOS platform variant
- Add macOS toggle button alongside Ubuntu Desktop and WSL2
- Add macOS hero badges, subtitle, stats bar, and terminal demo
- Add macOS screenshot showcase with terminal-style cards
- Add 'Built for macOS' section with Apple Silicon, Colima, Ghostty info
- Add macOS features grid, commands, and shortcuts sections
- Add macOS How It Works flow and FAQ section
- Add macOS footer description and data-macos-href on all platform links
- Update JS setPlatform() to use PLATFORMS allowlist for 3 platforms
- Add mobile-responsive CSS for 3-button toggle on small screens

Author: kevinevans1