fix(notifications): surface action failures as an error banner

When dismiss-notification or mark-all-read returns a failure the dropdown
now shows a localised error message above the list. Added
notification_error_generic key (de/en/es) as the fallback when the
action response carries no explicit error string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -522,6 +522,7 @@
 	"notification_filter_unread": "Ungelesen",
 	"notification_filter_mention": "Erwähnung",
 	"notification_filter_reply": "Antwort",
+	"notification_error_generic": "Aktion fehlgeschlagen. Bitte versuche es erneut.",
 	"notification_mark_all_read_aria": "Alle Benachrichtigungen als gelesen markieren",
 	"notification_load_more": "Ältere laden",
 	"notification_empty_history": "Keine Benachrichtigungen",