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.

backend/tests/test_demo_manifest.py

@@ -0,0 +1,55 @@
+from app.core.db import SessionLocal
+from app.seed_loader import reset_and_seed
+
+
+def test_demo_manifest_is_public(client):
+    # No login call at all -- the demo-entry screen and badge need this before any
+    # session exists.
+    response = client.get("/api/v1/demo/manifest")
+    assert response.status_code == 200
+
+
+def test_demo_manifest_shape(client):
+    body = client.get("/api/v1/demo/manifest").json()
+    assert body["organization_name"] == "Northstar Mobility"
+    assert body["demo_mode"] is True
+    assert body["synthetic_data"] is True
+    assert body["allow_reset"] is True
+    assert body["timezone"] == "Europe/Brussels"
+    assert body["guide_available"] is True
+    assert set(body["required_roles"]) == {"operations_manager", "rental_employee"}
+    assert body["last_reset_at"] is not None
+    assert body["anchor_date"] is not None
+
+    scenario_ids = {s["id"] for s in body["scenarios"]}
+    assert scenario_ids == {
+        "return-anomaly",
+        "duplicate-customer",
+        "booking-overlap",
+        "automation-retry",
+        "knowledge-question",
+    }
+    integration_keys = {i["key"] for i in body["integrations"]}
+    assert integration_keys == {"n8n", "ragcore", "mcp_hub"}
+
+
+def test_demo_manifest_scenarios_ready_after_fresh_reset(client):
+    db = SessionLocal()
+    try:
+        reset_and_seed(db)
+    finally:
+        db.close()
+
+    body = client.get("/api/v1/demo/manifest").json()
+    scenarios = {s["id"]: s for s in body["scenarios"]}
+    for scenario_id, scenario in scenarios.items():
+        assert scenario["ready"] is True, f"{scenario_id} should be ready right after a reset"
+        assert scenario["blocked_reason"] is None
+        assert scenario["start_path"]
+
+
+def test_demo_manifest_ragcore_labelled_as_demo_mode_not_live(client):
+    body = client.get("/api/v1/demo/manifest").json()
+    ragcore = next(i for i in body["integrations"] if i["key"] == "ragcore")
+    assert "Demomodus" in ragcore["status_label"]
+    assert "RAGcore" not in ragcore["status_label"]