feat: presentation demo mode — one-click private demo patient, screening on/off switch

- POST /auth/demo/fresh + «Демо: войти как Андрей» on the welcome screen: every sign-in gets a fresh private
  copy of veteran Andrey (consent, prescription, ЕМИАС record) and lands in the chat — no codes, nobody's
  crisis or half-done questionnaire leaks into another viewer.
- Initial screening switch: SCREENING_ENABLED env default + /demo/settings + a toggle in the demo console;
  off → after consent straight to the chat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dashboard/pages/2_Демо-консоль.py

@@ -33,6 +33,20 @@ w.render_connect_hero()
 w.show_flash()
 st.sidebar.caption(f"API: {w.get_api().base_url}")
 
+# One switch for the presentation: initial questionnaires after consent on/off (off → straight to the chat).
+try:
+    _screening = bool(w.get_api().demo_settings().get("screening_enabled", True))
+    _wanted = st.sidebar.toggle(
+        "Начальный тест (скрининг)",
+        value=_screening,
+        help="Выключено: после согласия пациент сразу попадает в чат, без опросников.",
+    )
+    if _wanted != _screening:
+        w.get_api().set_screening(_wanted)
+        st.sidebar.success("Начальный тест " + ("включён" if _wanted else "выключен"))
+except Exception:  # noqa: BLE001 — the switch must never break the console
+    st.sidebar.caption("Переключатель теста недоступен (API)")
+
 with st.container(border=True, key="panel-console-clock"):
     w.render_clock_bar(persona, key="console-clock")