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/Backends/index.tsx

@@ -216,11 +216,13 @@ export default function BackendsPage() {
       void qc.invalidateQueries({ queryKey: ['backends'] })
       void qc.invalidateQueries({ queryKey: ['domains'] })
     },
+    onError: (e: Error) => message.error(e.message),
   })
 
   const del = useMutation({
     mutationFn: async (id: number) => { await apiClient.delete(`/backends/${id}`) },
     onSuccess: () => { void qc.invalidateQueries({ queryKey: ['backends'] }) },
+    onError: (e: Error) => message.error(e.message),
   })
   const quickToggle = useMutation({
     mutationFn: async ({ id, row, checked }: { id: number; row: Backend; checked: boolean }) => {
@@ -453,7 +455,7 @@ function ServerPanel({ backendID }: { backendID: number }) {
     },
     { title: t('backends.server.weight'), dataIndex: 'weight', width: 80 },
     { title: t('backends.server.backup'), dataIndex: 'backup',
-      render: (v: boolean) => v ? <Tag color="purple">Backup</Tag> : '—', width: 100 },
+      render: (v: boolean) => v ? <Tag color="purple">{t('backends.server.backup')}</Tag> : '—', width: 100 },
     {
       title: t('backends.active'), dataIndex: 'active', width: 80,
       render: (v: boolean, row: BackendServer) => (