feat(demo): add demo manifest, Dutch demo entry, permanent badge and About page

Adds GET /api/v1/demo/manifest as a single source of truth for the demo's
fictional org identity (Northstar Mobility -- surfacing the project's
already-locked tenant name), synthetic-data/reset state, and live scenario
readiness. Rewrites the login screen in Dutch with an honest, no-password
demo entry and a guided-demo entry point, replaces the loud full-width
demo banner with a subtle badge + popover, and adds a compact About page
explaining what's real vs. synthetic vs. not yet connected.

frontend/src/context/DemoManifestContext.tsx

@@ -0,0 +1,52 @@
+import { createContext, useContext, useEffect, useState, type ReactNode } from "react";
+import { api } from "../api/client";
+import type { DemoManifest } from "../api/types";
+
+interface DemoManifestState {
+  manifest: DemoManifest | null;
+  loading: boolean;
+  refresh: () => void;
+}
+
+const DemoManifestContext = createContext<DemoManifestState | undefined>(undefined);
+
+export function DemoManifestProvider({ children }: { children: ReactNode }) {
+  const [manifest, setManifest] = useState<DemoManifest | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [version, setVersion] = useState(0);
+
+  useEffect(() => {
+    let cancelled = false;
+    setLoading(true);
+    // Public endpoint by design: the demo-entry screen needs this before any session
+    // exists, so it is never gated behind auth.
+    api
+      .get<DemoManifest>("/api/v1/demo/manifest")
+      .then((result) => {
+        if (!cancelled) setManifest(result);
+      })
+      .catch(() => {
+        if (!cancelled) setManifest(null);
+      })
+      .finally(() => {
+        if (!cancelled) setLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [version]);
+
+  return (
+    <DemoManifestContext.Provider
+      value={{ manifest, loading, refresh: () => setVersion((v) => v + 1) }}
+    >
+      {children}
+    </DemoManifestContext.Provider>
+  );
+}
+
+export function useDemoManifest(): DemoManifestState {
+  const ctx = useContext(DemoManifestContext);
+  if (!ctx) throw new Error("useDemoManifest must be used within DemoManifestProvider");
+  return ctx;
+}