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>

web/src/api/client.ts

@@ -44,6 +44,8 @@ export const api = {
   signInEmias: (code: string) => post<AuthResult>("/auth/emias", { code }),
   signInEmiasDemo: (personaToken: string) => post<AuthResult>("/auth/emias/demo", { persona_token: personaToken }),
   signInAnonymous: (focus?: Focus) => post<AuthResult>("/auth/anonymous", { focus }),
+  // Presentation entry: a fresh private copy of the demo patient, straight to the chat.
+  signInDemoFresh: () => post<AuthResult>("/auth/demo/fresh", {}),
   patientCapabilities: () => request<{ primary_screening: boolean }>("/patient/capabilities"),
   connectDoctor: (token: string) => post<{ has_doctor: boolean }>(`/me/${encodeURIComponent(token)}/doctor/connect`),
   demoPersonas: () => request<DemoPersona[]>("/patient/demo-personas"),

web/src/content/ru.ts

@@ -41,6 +41,12 @@ export const ru = {
   },
   welcome: {
     title: "Вход",
+    demo: {
+      title: "Демо: войти как Андрей",
+      text: "Ветеран СВО, 34 года, сертралин. Назначения и карта ЕМИАС уже подключены — сразу в чат.",
+      busy: "Готовим демо…",
+      error: "Не удалось открыть демо. Попробуйте ещё раз.",
+    },
     emias: {
       title: "Войти через ЕМИАС",
       perks: [

web/src/screens/Welcome.tsx

@@ -1,5 +1,9 @@
+import { useState } from "react";
 import { useNavigate } from "react-router-dom";
 
+import { api } from "../api/client";
+import { useSession } from "../session/SessionContext";
+
 import { telHref, useHelp } from "../components/HelpSheet";
 import { Icon, LogoMark } from "../components/Icon";
 import { ru } from "../content/ru";
@@ -7,10 +11,26 @@ import { ru } from "../content/ru";
 // A1 · Вход: без сайдбара, одна карточка по центру. ЕМИАС — основной путь, анонимный — по желанию
 export function Welcome() {
   const navigate = useNavigate();
+  const { signIn } = useSession();
+  const [demoBusy, setDemoBusy] = useState(false);
+  const [demoError, setDemoError] = useState(false);
   const { resources, open } = useHelp();
   const t = ru.welcome;
   const emergency = resources[0];
 
+  // Presentation: one click → a fresh private demo patient → the chat.
+  const demo = async () => {
+    setDemoBusy(true);
+    setDemoError(false);
+    try {
+      signIn(await api.signInDemoFresh());
+      navigate("/messages", { replace: true });
+    } catch {
+      setDemoError(true);
+      setDemoBusy(false);
+    }
+  };
+
   return (
     <div className="signin-page">
       <main className="signin" id="main">
@@ -25,6 +45,28 @@ export function Welcome() {
         <section className="signin__card">
           <h1 className="h1">{t.title}</h1>
 
+          <button type="button" className="entry entry--emias" onClick={demo} disabled={demoBusy}>
+            <span className="entry__top">
+              <span className="tile-icon tile-icon--teal">
+                <Icon name="check" size={26} />
+              </span>
+              <span className="entry__title">
+                {demoBusy ? t.demo.busy : t.demo.title}
+                <Icon name="chevronRight" />
+              </span>
+            </span>
+            <span className="muted small">{t.demo.text}</span>
+          </button>
+          {demoError && (
+            <p className="muted small center" role="alert">
+              {t.demo.error}
+            </p>
+          )}
+
+          <div className="divider" role="separator">
+            <span>{t.or}</span>
+          </div>
+
           <button type="button" className="entry entry--emias" onClick={() => navigate("/emias")}>
             <span className="entry__top">
               <span className="tile-icon tile-icon--teal">