fix(ui): fehlende onError-Handler + i18n in Backends/Domains/Dashboard

- Backends: create + del Mutations bekommen onError; 'Backup'-Tag → t()
- Domains: create + del Mutations bekommen onError
- Dashboard: LicenseChip-Texte ('License OK', 'Trial') in t() überführt;
  useTranslation() direkt in der Komponente verwendet
- en/de: dashboard.licenseOk/licenseTrial/licenseTrialDays ergänzt

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

management-ui/src/i18n/locales/de/common.json

@@ -773,7 +773,10 @@
       "free": "frei",
       "conntrack": "Conntrack",
       "uptime": "Uptime"
-    }
+    },
+    "licenseOk": "Lizenz OK",
+    "licenseTrial": "Trial",
+    "licenseTrialDays": "Trial · {{days}}d"
   },
   "ntp": {
     "title": "Zeitserver (Chrony)",