feat(site): add interactive API playground and redesign homepage

- 'Try it' playground: resource select + slug autocomplete via datalist,
  Send/Random buttons, syntax-highlighted JSON response, copy-as-curl
- Featured device cards built from detail responses, with overall-score
  conic-gradient ring and per-category bars; click to load in playground
- Sticky nav, animated stat counters, redesigned hero, 'how it works' grid
- Graceful image fallback (image CDN currently 404s for all entries) so
  cards remain clean without device photos
- All client-side fetch against the existing static JSON dump; no server
  or hosting change required

Author: Seungpyo1007