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/pages/Domains/index.tsx

@@ -167,6 +167,7 @@ export default function DomainsPage() {
       form.resetFields()
       void qc.invalidateQueries({ queryKey: ['domains'] })
     },
+    onError: (e: Error) => message.error(e.message),
   })
 
   const del = useMutation({
@@ -176,6 +177,7 @@ export default function DomainsPage() {
     onSuccess: () => {
       void qc.invalidateQueries({ queryKey: ['domains'] })
     },
+    onError: (e: Error) => message.error(e.message),
   })
   const quickToggle = useMutation({
     mutationFn: async ({ id, row, checked }: { id: number; row: Domain; checked: boolean }) => {