Two UI regressions from the Vue->React migration:

- The cell selection overlay (green border box, editor input, and
  fill-handle square) is positioned with content-box math, but Tailwind's
  preflight applies `box-sizing: border-box` globally. The borders ate into
  the declared width/height, so the green box rendered too small and no
  longer wrapped the selected cell. Explicitly set `box-sizing: content-box`
  on the editor overlay elements to restore the original sizing.

- The native color inputs (Text, Fill, Border color) rendered as bulky
  black boxes. Style them as compact rounded swatches with a subtle border
  and hover accent, matching the original look.

Rebuilds the GitHub Pages bundle so the fixes ship to the live site.