fix: auto-reconnect to usbguard-dbus after restart; polkit dialog no longer breaks on re-setup

[Copilot]

Re-running sudo make setup-usbguard (or reinstalling without uninstalling first) calls systemctl restart usbguard-dbus, which gives the daemon a new unique D-Bus name. The Python daemon's connect_to_signal match rule was pinned to the old name — silently receiving nothing, killing the polkit dialog permanently until the user service was manually restarted.

Changes

• src/usb-auth-guard — reconnect on org.usbguard1 owner changes via bus.watch_name_owner:
  • On disappear: mark state['iface'] = None
  • On reappear: re-create proxy and re-call connect_to_signal against the new unique name
  • Replaced lazy startup check with explicit get_name_owner() for fast-fail if usbguard-dbus is not running

def on_usbguard_name_changed(new_owner):
    if new_owner and state['iface'] is None:
        subscribe()          # fresh proxy → valid match rule against new unique name
    elif not new_owner and state['iface'] is not None:
        state['iface'] = None  # mark stale, await reappear

bus.watch_name_owner('org.usbguard1', on_usbguard_name_changed)

• Makefile setup-usbguard — after restarting usbguard services, runs systemctl --user -M <uid>@ restart usb-auth-guard for all logged-in users as a belt-and-suspenders recovery path for daemons running the pre-fix binary.