Update gnexus-ui-kit to 1.0.0, drop vendored GnModal, use kit tokens ...

Upgrade the git dependency to 1.0.0 (allow-git for npm 12, recorded in
frontend/.npmrc). The vendored GnModal.vue copy is no longer needed: the
kit's overlay transitions are fixed upstream via useOverlayTransition,
and the official component is API-identical. Replace the five hardcoded
palette hexes in styles.css with kit design tokens so theme swaps stay
one import away.

Also restate the form-group hint styles for .tag-input: GnTagInput
renders its hint outside .form-group, so the scoped .form-group
.input-info styles from kit 1.0 miss it and the hint was glued to the
field with no top margin.

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

Update gnexus-ui-kit to 0.4.0 ...

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

Implement initial gnexus-creds MVP scaffold