feat(react): add themed workshop coverage (#55)

## Summary

- add Neutral, Monochrome, Emerald, Samva, Dark Plus, Solarized, and
Tokyo Night presets
- keep theme and color-scheme controls in the workshop's global top
toolbar
- keep story-specific controls in the full-width bottom panel
- cover fonts, dropdown stacking, Add record visibility, panel layout,
and host lifecycle flows

## Validation

- 59 React unit tests
- 4 package artifact tests
- 24 Playwright browser tests across workshop flows and themes
- repository lint, format, and diff checks

Local visual flows were exercised against the isolated workshop server;
no existing docs server was restarted.