Add Spoolman URL setup to settings modal

- New POST /api/ui/set_spoolman_url endpoint writes spoolman_url to
  config.json at runtime — no file editing or reinstall needed
- Settings modal always shows a Spoolman URL input; the sync mode
  options only appear once a URL is saved
- Input is pre-filled from state on each render (skipped while modal
  is open to avoid clobbering in-progress edits)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>