session-extend: preempt previous WaitExtendAuthSession on new wait

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.

client/server/server.go

@@ -1539,8 +1539,21 @@ func (s *Server) WaitExtendAuthSession(
 		return nil, gstatus.Errorf(codes.InvalidArgument, "invalid device code or no active extend-session flow")
 	}
 
-	tokenInfo, err := oAuthFlow.WaitToken(ctx, authInfo)
+	// Preempt a previous WaitExtendAuthSession (e.g. when the tray
+	// notification and the about-to-expire dialog both start a flow on
+	// the same deadline). The older waiter exits via context.Canceled;
+	// the new one takes over the IdP poll.
+	s.extendAuthSessionFlow.CancelWait()
+
+	waitCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	s.extendAuthSessionFlow.SetWaitCancel(cancel)
+
+	tokenInfo, err := oAuthFlow.WaitToken(waitCtx, authInfo)
 	if err != nil {
+		if errors.Is(err, context.Canceled) {
+			return nil, gstatus.Errorf(codes.Canceled, "extend-session flow preempted")
+		}
 		return nil, gstatus.Errorf(codes.Internal, "failed to obtain JWT token: %v", err)
 	}