[client] Revert silent deadline-rejected event; restore userMessage

MetaSessionDeadlineRejected had no UI consumer: the tray only does
metadata-driven localisation for MetaSessionWarning events; all other
SystemEvents display userMessage directly (tray_events.go). Leaving
userMessage empty made the rejection invisible to the user.

Restore the English userMessage so the generic event path shows
something, and remove the unused MetaSessionDeadlineRejected constant.

https://claude.ai/code/session_01Y3bQoNgcVjTD4zDTvv7a8u

client/internal/auth/sessionwatch/event.go

@@ -33,13 +33,6 @@ const (
 	// for the T-10 event, FinalWarningLead for the T-2 event) so the UI
 	// can show "expires in ~N minutes" without hardcoding either constant.
 	MetaSessionLeadMinutes = "lead_minutes"
-	// MetaSessionDeadlineRejected is set to the rejection reason when the
-	// daemon discards a deadline received from the management server (e.g.
-	// pre-epoch, too far in the future, or past the clock-skew tolerance).
-	// The UI uses this key to detect the event and display a localized
-	// message; userMessage is intentionally left empty for the same reason
-	// as the warning events above.
-	MetaSessionDeadlineRejected = "session_deadline_rejected"
 )
 
 // expiresAtLayout is the wire format used for MetaSessionExpiresAt.

client/internal/engine_authsession.go

@@ -10,7 +10,6 @@ import (
 	"google.golang.org/protobuf/types/known/timestamppb"
 
 	cProto "github.com/netbirdio/netbird/client/proto"
-	"github.com/netbirdio/netbird/client/internal/auth/sessionwatch"
 	"github.com/netbirdio/netbird/client/system"
 )
 
@@ -57,8 +56,8 @@ func (e *Engine) ApplySessionDeadline(ts *timestamppb.Timestamp) {
 			cProto.SystemEvent_ERROR,
 			cProto.SystemEvent_AUTHENTICATION,
 			"session deadline rejected",
-			"",
-			map[string]string{sessionwatch.MetaSessionDeadlineRejected: err.Error()},
+			"The session expiry time from the server could not be applied. Re-login may be required.",
+			map[string]string{"reason": err.Error()},
 		)
 	}
 }