- style.css with colour, type and spacing tokens; light grey chrome around
  the 3D view, one petrol accent for the file button, checkboxes, focus and
  a progress line along the header (replaces the native progress bar)
- header with title; the file input is a button (native input visually
  hidden, keyboard focus ring only for keyboard use)
- sidebar panel "Level of detail" with hover rows; panel message tells
  missing file, loading and files without geometry apart
- viewer: invitation to open a file while nothing is shown, mouse controls
  hint once there is geometry; viewer background #EDF3F4 matches the palette
- narrow screens: panels above the view

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>