- Register Navi themes as Textual themes so $tui-* CSS variables resolve.

- Update ChatPanel, StatusPanel, InputBox to use palette colors.

- Update all content renderers to draw from the active theme.

- Add set_active_theme/get_active_theme helpers for runtime color access.

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

Add selection (magenta) and link (teal) for cursor line, active selection and URLs.

Tests still green.

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

- 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>