fix(ui): backup schedule shows a load error with retry, not "not configured"

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/app/(app)/settings/backup/page.tsx

@@ -22,6 +22,7 @@ import { Button } from "@/components/ui/button";
 import { Label } from "@/components/ui/label";
 import { PageHeader } from "@/components/page-header";
 import { ConfirmDialog } from "@/components/confirm-dialog";
+import { ErrorState } from "@/components/query-state";
 import {
   Select,
   SelectContent,
@@ -313,6 +314,12 @@ function ScheduleSection() {
       <CardContent className="flex flex-col gap-4 text-sm">
         {scheduleQuery.isLoading ? (
           <p className="text-muted-foreground">Lädt…</p>
+        ) : scheduleQuery.isError ? (
+          <ErrorState
+            what="Zeitplan-Angaben"
+            onRetry={() => void scheduleQuery.refetch()}
+            isRetrying={scheduleQuery.isFetching}
+          />
         ) : !status?.enabled ? (
           <p className="text-muted-foreground">
             Automatische Backups sind noch nicht konfiguriert - weder per Umgebungsvariable noch