When the tray "Extend now" notification action and the about-to-expire
dialog both start a flow for the same deadline, the daemon was running
two independent IdP polls and the older one surfaced an InvalidArgument
toast as soon as the second RequestExtend overwrote the pending flow.
Follow the WaitSSOLogin pattern: at the top of WaitExtendAuthSession
cancel the previous wait (the SetWaitCancel/CancelWait pair on
PendingFlow already existed but was unused), then register the new
wait's cancel. Preempted callers exit with codes.Canceled; the
authsession service translates that into ExtendResult{Preempted: true}
so the tray and the React dialog can stay silent on the losing flow
instead of showing a false-failure toast / error dialog.
Adds an end-to-end SSO session-extension feature: the management server
publishes per-peer session deadlines on every Login/Sync, a new
ExtendAuthSession RPC refreshes the deadline using a fresh JWT without
tearing down the tunnel, and the daemon tracks the deadline locally so
the UI can fire a T-10min warning toast with an interactive "Extend now"
action.
