fix(admin): surface query errors instead of an infinite spinner

P19. `useSystemStatus()` and `useAdminSettings()` already throw on
non-OK responses so React Query sets `isError`, but neither caller
read it — the system-status panel just kept spinning forever and the
settings sections silently rendered defaults as if they were live.

Now system-status renders a destructive inline alert when isError, and
the admin page top renders a one-line "Failed to load admin settings"
banner when the shared settings query fails so the admin notices the
toggles below aren't reflecting reality. Both messages have new i18n
keys (`admin.systemStatusLoadError`, `admin.adminSettingsLoadError`)
in en.json (canonical) + de.json.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

messages/de.json

@@ -1259,6 +1259,8 @@
     "activeTokens": "Aktive API-Tokens",
     "activeSessions": "Aktive Sessions",
     "loadingStatus": "Lade Status...",
+    "systemStatusLoadError": "System-Status konnte nicht geladen werden. Bitte die Seite neu laden.",
+    "adminSettingsLoadError": "Admin-Einstellungen konnten nicht geladen werden. Schalter unten zeigen evtl. nicht den Live-Zustand — Seite neu laden.",
     "workerStatus": "Job-Worker",
     "workerRunning": "Aktiv",
     "workerStopped": "Gestoppt",

messages/en.json

@@ -1259,6 +1259,8 @@
     "activeTokens": "Active API Tokens",
     "activeSessions": "Active Sessions",
     "loadingStatus": "Loading status...",
+    "systemStatusLoadError": "Failed to load system status. Try refreshing the page.",
+    "adminSettingsLoadError": "Failed to load admin settings. Toggles below may not reflect the live state — refresh the page.",
     "workerStatus": "Job Worker",
     "workerRunning": "Running",
     "workerStopped": "Stopped",

src/app/admin/page.tsx

@@ -16,10 +16,16 @@ import { SystemStatusSection } from "@/components/admin/system-status-section";
 import { UmamiSection } from "@/components/admin/umami-section";
 import { UserManagementSection } from "@/components/admin/user-management-section";
 import { WebPushVapidSection } from "@/components/admin/web-push-vapid-section";
+import { useAdminSettings } from "@/components/admin/_shared";
 
 export default function AdminPage() {
   const { user } = useAuth();
   const { t } = useTranslations();
+  // P19: surface admin-settings fetch failures once at the top of the
+  // page. Many child sections share `useAdminSettings()` and render
+  // defaults silently on error — the banner makes the failure visible
+  // so the admin knows the toggles below aren't reflecting real state.
+  const { isError: settingsError } = useAdminSettings();
 
   if (!user || user.role !== "ADMIN") return null;
 
@@ -32,6 +38,15 @@ export default function AdminPage() {
         <p className="text-muted-foreground text-sm">{t("admin.subtitle")}</p>
       </div>
 
+      {settingsError && (
+        <div
+          role="alert"
+          className="text-destructive bg-destructive/10 border-destructive/30 rounded-md border px-3 py-2 text-sm"
+        >
+          {t("admin.adminSettingsLoadError")}
+        </div>
+      )}
+
       <StatusCardGrid />
 
       <div className="space-y-6">

src/components/admin/system-status-section.tsx

@@ -22,7 +22,7 @@ import { StatusItem, useSystemStatus } from "./_shared";
 export function SystemStatusSection({ id }: { id: string }) {
   const { t } = useTranslations();
   const fmt = useFormatters();
-  const { data: status } = useSystemStatus();
+  const { data: status, isError } = useSystemStatus();
 
   return (
     <div
@@ -142,6 +142,16 @@ export function SystemStatusSection({ id }: { id: string }) {
             />
           )}
         </div>
+      ) : isError ? (
+        // P19: surface load failures inline instead of leaving the
+        // section spinning forever. The hook throws on non-OK
+        // responses, so isError is true on 500s, network errors, etc.
+        <div
+          role="alert"
+          className="text-destructive bg-destructive/10 border-destructive/30 mt-4 rounded-md border px-3 py-2 text-sm"
+        >
+          {t("admin.systemStatusLoadError")}
+        </div>
       ) : (
         <div className="mt-4 flex items-center gap-2">
           <Loader2 className="text-muted-foreground h-4 w-4 animate-spin" />