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>

Sync tokens and components with Figma variables and frames ...

- Update primitive palette: add neutral-25, adjust warning-bg to #FEF3E5.
- Add full radius scale (xs/sm/md/lg/xl/2xl/full) from radius.tokens.json.
- Update semantic surfaces/borders from Mode 1 tokens.
- Realign buttons: primary dark, secondary mint, accent yellow, tertiary ghost.
- Realign badges, alerts, cards, inputs, checkbox, radio, switch, modal, toast.
- Update demo palette/utilities and style-guide docs.
- Add exported Figma frames and variables under 'figma design parts/' for reference.

Add Input, Badge, Alert components and foundation demos ...

- Add src/scss/components/_forms.scss with input, label, help text,
  and error/success/warning states.
- Add src/scss/components/_badges.scss and _alerts.scss with semantic
  variants and outline/size options.
- Add GnInput, GnBadge, GnAlert Vue adapters mirroring gnexus-ui-kit API.
- Update plugin and entry exports for new components.
- Add demo partials for Typography, Palette, Utilities, Inputs, Badges,
  and Alerts, and include them in demo/index.html.
- Extend Vue example app to render new components.