v0.4.0 — GnRepeater, redesigned checkbox/radio ...

- Add GnRepeater Vue component for dynamic field groups
- Redesign checkbox and radio styles to match kit aesthetic:
  hard-panel background, accent borders, checkmark/dot indicators,
  hover → primary border, checked → subtle tint + colored mark
- Bump version to 0.4.0

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

Replace translucent $surface-panel-muted with solid $color-dark ...

$surface-panel-muted was `rgba($color-text-light, 0.045)` — an ugly,
almost-invisible blue tint. It is now `=#1F2335`, the proper dark panel
background for cards, groups, tables, tabs, drawers, etc.

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

Modernize Sass build pipeline

Add Tokyo Night color pass

Scaffold GNexus UI kit