style: polish demo visuals and add screenshot workflow ...

- Replace flat palette swatches with color cards (name + hex).
- Increase vertical spacing for demo-section and demo-row.
- Fix Description List to show term when items use { key, value }.
- Style advanced-select input like a regular input with a custom arrow.
- Resize file attachment cards from 420px to 160px, fix grid gaps and padding.
- Make demo background fixed with a subtle parallax drift on scroll.
- Link demo.css in demo/index.html so demo styles actually load.
- Add tests/demo-screenshot.spec.js and npm run screenshots for regular
  visual review.

All 19 e2e tests pass.

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