Fix SET_ACTIVE_SPOOL not firing on service restart; add spoolman_mode to installer

- Track active slot in-process (_ws_active_slot sentinel) instead of reading
  from disk state, so SET_ACTIVE_SPOOL fires even when restarting mid-print
  with the same spool already active
- install.sh: prompt for spoolman_mode (direct/moonraker) when a Spoolman URL
  is provided, and write it to data/config.json

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