On macOS app.quit() closes windows but window-all-closed deliberately keeps
the process alive (Dock convention). Every detached hand-off (update swap,
relaunch, Windows bootstrap recovery, uninstall cleanup) waits for the
desktop PID to exit before replacing/removing the bundle — so the process
never dying means the script spins its full PID-wait and the user sees a
blank app, or an uninstall that appears to do nothing.
Add a module-level isQuittingForHandoff flag, set before every hand-off
app.quit(); window-all-closed then quits on all platforms when it's set.
Covers all five hand-off sites including the Linux relaunch path.
376d021fee