[client] Localize session deadline rejected notification via metadata key

Follow the same pattern as session-warning events: the daemon emits an
empty userMessage and puts the signal in a typed metadata key
(MetaSessionDeadlineRejected); the UI tray detects the key and builds a
locale-aware OS notification from i18n strings.

Changes:
- sessionwatch/event.go: add MetaSessionDeadlineRejected constant
- engine_authsession.go: empty userMessage, use the new metadata key
- ui/authsession/warning.go: re-export MetaDeadlineRejected for UI consumers
- ui/tray_events.go: gate on isDeadlineRejected alongside isSessionWarning;
  new branch calls t.notify with localized title/body
- i18n locales (en/de/hu): add notify.sessionDeadlineRejected.{title,body}

https://claude.ai/code/session_01Y3bQoNgcVjTD4zDTvv7a8u

client/ui/authsession/warning.go

@@ -19,10 +19,11 @@ import (
 // side) so UI-side consumers don't have to import the daemon-internal
 // package directly.
 const (
-	MetaWarning     = sessionwatch.MetaSessionWarning
-	MetaFinal       = sessionwatch.MetaSessionFinal
-	MetaExpiresAt   = sessionwatch.MetaSessionExpiresAt
-	MetaLeadMinutes = sessionwatch.MetaSessionLeadMinutes
+	MetaWarning          = sessionwatch.MetaSessionWarning
+	MetaFinal            = sessionwatch.MetaSessionFinal
+	MetaExpiresAt        = sessionwatch.MetaSessionExpiresAt
+	MetaLeadMinutes      = sessionwatch.MetaSessionLeadMinutes
+	MetaDeadlineRejected = sessionwatch.MetaSessionDeadlineRejected
 )
 
 // Warning is the typed payload emitted on the session-warning Wails