Fix v0.4.0 review findings and demo asset paths ...

- Restore visible hover feedback on radio/checkbox (background $color-grey)
- Drop dead checked/hover border-color rules; guard checked with :not(:disabled)
- Tokenize radio border as $border-width-choice and add $range-thumb-size
  so the choice-size bump no longer widens range slider thumbs
- Add cursor: pointer to switch wrapper
- Revert escaped doc snippets to absolute URLs (relative sweep was
  teaching fake ./routes in documentation samples)
- Add compare-heights.js demo sync check with CHROME_PATH fallback;
  ignore screenshots/

Scaffold GNexus UI kit