Adds a Shutdown entry after the Worker menu's Restart/Update Claude CLI
items. Confirms via IDialogService.ConfirmAsync, then kills this
installation's ClaudeDo.Worker process(es) (shared kill loop, extracted
from RestartWorkerService) and force-closes the window through the same
_forceClose path MainWindow already uses for the Mission-Control close
confirm, so no second confirmation dialog shows.
