- while nothing is shown, the whole view is a button that opens the file
  dialog ("Open a CityGML file / or drop it here")
- files can be dropped on the view at any time; an overlay shows the drop
  target, a drop replaces the shown file like choosing one
- drops elsewhere on the page are ignored instead of making the browser
  leave the viewer

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

- 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>