On error/manual outcomes stop_ui('leave-window') kept the browser shim
window open indefinitely, so an aborted update left a Chrome window on
screen until the user closed it by hand; repeated update attempts piled
up more windows.
stop_ui now always closes the shim. leave-window paths keep it up for a
short grace period (HERMES_UPDATE_SHIM_GRACE_SECONDS, default 15) so a
watching user can read the message, then close it. The success path is
unchanged. The error/manual outcome is durably written to
.hermes-update-result.json and surfaced in a dialog on the next Desktop
boot, so closing the shim loses no information.
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