fix: responsive Power of Two — adapts to any screen size

- Layout computed from actual canvas dimensions (window.innerWidth/Height)
- All positions/sizes relative via rx()/ry()/rf() helpers
- Button grid auto-scales, digit row adapts to width
- Font sizes proportional to screen width
- Touch hit-testing matches rendered button positions

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