fix(updater): print-aware unattended updates, reliable boot provisioning and self-heal, gapless restart splash

[gmmcosta15]

Description

• Feature
• Bug fix
• Code refactor
• Documentation

Hardens the updater's boot provisioning, boot heal and update overlay (defects found on the RF50), pauses unattended updater work during prints, and removes the black screen on UI restart.

• Printing guard: the daemon reads Klipper print_stats.state from klippy.sock (any error counts as idle, as in Moonraker). While printing or paused, it defers boot installs, fetches, apt refreshes, forward heal and background apt.
• Provisioning: missing components install once per start, after the boot heal. Offline devices skip silently and re-check for about 30 min, then wait for Update. A failed install removes the clone and disables its unit (new sudoers rule), and is skipped until Update.
• Spoolman: the health check fails fast, the hook's service is never started twice, and the hook fails after 120 s unhealthy before it edits moonraker.conf.
• Boot heal: retried instead of skipped while the lock is held, and run first by every lock holder (daemon and CLI). The start script's crash-loop rollback stands down while the daemon's self-heal is active.
• Daemon: no background apt when a restart is pending (dpkg is never SIGKILLed). Shutdown drains tasks for at most 60 s. Non-executable hooks fail clearly, and fetch failures log at ERROR.
• Install: the D-Bus activation file gets Exec=/bin/false, so the UI can revive a stopped daemon. The hook and post-merge re-run the install on any install-file change. install-updater.sh re-enables BlocksScreen.service.
• UI: a "Missing component, installing ..." overlay without Cancel, held through the UI restart until fresh status arrives. Busy state is replayed on connect, a toast appears if the 400 s safety net fires, and versions show as tag+N.
• Restart splash: feh paints the splash on the X root at X start and in ExecStop before SIGTERM, so the ~14.5 s window teardown no longer shows a black screen.