- Extract exact gnexus-ui-kit cyberpunk palette from
  webclient/vendor/gnexus-ui-kit/src/scss/_palette-colors.scss.
- Add clients/terminal/tui/themes.py with Theme dataclass, ThemeRegistry,
  and gnexus-dark / gnexus-light themes.
- Wire theme into NaviCodeTui via _apply_theme() on mount; default is
  gnexus-dark.

Tests: 463 passed, 1 skipped (excluded unrelated websocket test).

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