Style login card: 460px max-width, max-content submit, centered form-group ...

- .card.login-card max-width 360px -> 460px
- .login-card-submit width 100% -> max-content
- Fix .form-group indentation inside .login-card
- Demo login button: btn-secondary with-icon ph-sign-in

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add GnLoginCard component for standardized login forms ...

- Create GnLoginCard Vue adapter component with username/password
  fields, optional remember-me checkbox, submit button, and
  forgot-password / sign-up links.
- Add `.login-card` styles in _cards.scss with centered header,
  full-width submit button, and spaced link row.
- Register GnLoginCard in plugin and index exports.
- Add login-card demo block to demo/partials/cards.html.
- Update CLAUDE.md, README, and Vue docs with component entry.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Replace code-example-copy with standard btn-icon ...

All copy buttons inside `.code-example` blocks now use `.btn-icon.btn-icon-sm`
with a Phosphor `ph-copy` icon instead of the old text-based `.code-example-copy`.

Updated `code-examples.js` to swap the icon to `ph-check` for 1.2 s after copy,
matching the behaviour of `GnCopyButton`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add horizontal card to demo page ...

- Insert horizontal card example into demo/partials/cards.html
- Add HTML and Vue code examples for copy-paste
- Rebuild dist

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add card and table variants

Generalize UI kit branding

Scaffold GNexus UI kit