feat: live agent answers (SSE streaming) + «Чат» tab on phones

- POST /dialog/{token}/events/stream: Server-Sent Events with the agent's steps («Смотрю вашу карту ЕМИАС…»,
  «Ищу в клинических рекомендациях…»), the answer as the model writes it (OpenRouter stream=true), then the
  final replies (same as /events, incl. the forbidden-output filter and sources). X-Accel-Buffering: no.
- Web: free-text messages use the stream — a live bubble with steps, typing dots and a cursor; falls back to
  the plain request if the stream breaks.
- Phones: the bottom tab bar gets «Чат» (it had no way back to the chat).

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

web/src/components/AppShell.tsx

@@ -16,6 +16,7 @@ import { Icon, LogoMark, type IconName } from "./Icon";
 
 const TABS: { to: string; label: string; icon: IconName }[] = [
   { to: "/today", label: ru.nav.today, icon: "home" },
+  { to: "/messages", label: ru.nav.chat, icon: "message" },
   { to: "/meds", label: ru.nav.meds, icon: "pill" },
   { to: "/doctor", label: ru.nav.doctor, icon: "doctor" },
   { to: "/profile", label: ru.nav.profile, icon: "user" },