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>

Add Divider, Accordion, Stepper and FileAttachment components ...

- Implement CSS for divider, accordion, stepper and file-upload/attachment.
- Add browser Accordion helper and expose it on window.VMKUIKit.
- Add Vue adapters: GnAccordion, GnSteps, GnFileUpload, GnFileAttachment.
- Add demo partials and figma match pages for divider and attached-file.
- Fix Figma comparison flattening for transparent baselines (e.g. breadcrumbs, attached-file).
- Add vertical spacing to the demo index page to separate sections.

All Figma visual tests and the Vue adapter smoke build pass.