[client] Leave userMessage empty on deadline-rejected event; use metadata key

Daemon-layer PublishEvent userMessage strings are not localized — the UI
reads metadata keys and builds its own locale-aware copy (same pattern
as the session-warning events in event.go). Drop the hardcoded English
sentence from the deadline-rejected event and instead surface the
rejection reason via a new MetaSessionDeadlineRejected metadata key so
the UI can detect and localize it.

https://claude.ai/code/session_01Y3bQoNgcVjTD4zDTvv7a8u

client/internal/auth/sessionwatch/event.go

@@ -33,6 +33,13 @@ 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.