feat(demo): login trigger, feature gating + admin UI (HU/EN/DE)

Login page quietly probes /auth/demo (debounced) as a valid email is
typed/pasted and reloads into the app on a match — no visible button.
Demo sessions default to the whole library, get a one-time shared-account
warning, never see the YouTube-connect onboarding/access UI or sync
actions, and admins get a demo whitelist + reset panel in Settings.

frontend/src/App.tsx

@@ -189,6 +189,17 @@ export default function App() {
         message: t("common.accessRequestsMessage", { count: pending }),
       });
     }
+    // The demo account is shared: there are no subscriptions, so default it to the whole
+    // library (the "my" feed would be empty), and warn that its state is communal.
+    if (meQuery.data?.is_demo) {
+      setFilters({ ...loadStoredFilters(), scope: "all" });
+      notify({
+        level: "warning",
+        title: t("common.demoTitle"),
+        message: t("common.demoSharedNotice"),
+        requiresInteraction: true,
+      });
+    }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [meQuery.data?.id]);