improvement(demo): permanent banner instead of a re-popping toast

The shared-account warning was a requiresInteraction toast that re-fired
on every reload. Replace it with a permanent, non-dismissible banner (like
the version banner) shown above the content while in the demo account.

frontend/src/components/DemoBanner.tsx

@@ -0,0 +1,17 @@
+import { useTranslation } from "react-i18next";
+import { AlertTriangle } from "lucide-react";
+
+// Permanent, non-dismissible bar shown to the shared demo account so it's always clear the
+// state is communal. (Replaces the old login-time toast, which re-popped on every reload.)
+export default function DemoBanner() {
+  const { t } = useTranslation();
+  return (
+    <div className="shrink-0 flex items-center gap-2 px-4 py-2 text-sm bg-amber-500/15 border-b border-amber-500/30 text-fg">
+      <AlertTriangle className="w-4 h-4 text-amber-500 shrink-0" />
+      <span className="min-w-0">
+        <span className="font-semibold">{t("common.demoTitle")}</span>
+        <span className="text-muted"> — {t("common.demoSharedNotice")}</span>
+      </span>
+    </div>
+  );
+}