Stage 15: dual Telegram bots & language-gated variants

Service-agnostic refinement of the owner's idea: the sign-in service returns a
set of supported game languages with the user identity, and the lobby gates the
New Game variant choice by it (en -> English; ru -> Russian + Эрудит).

- Connector hosts two bots in one container (one per service language, each its
  own token + game channel; the same telegram_id spans both). ValidateInitData
  tries each token and returns the validating bot's service_language +
  supported_languages. Per-language config (TELEGRAM_BOT_TOKEN_EN/_RU, channels).
- supported_languages rides the Session (fbs, session-scoped, not persisted); the
  UI offers only the matching variants on New Game — gating only the START of a
  new game (auto-match + friend invite), not accept/open/play; backend does not
  enforce.
- service_language persisted (accounts.service_language, migration 00010, written
  every login, last-login-wins) and routes the user-facing Notify push back
  through the right bot (push-target coalesces with preferred_language).
- Admin SendToUser/SendToGameChannel gain an operator-chosen language selector in
  the console (unrelated to ValidateInitData).
- Non-Telegram logins carry the gateway default set
  (GATEWAY_DEFAULT_SUPPORTED_LANGUAGES, all variants).

Wire (committed regen): ValidateInitDataResponse +service_language
+supported_languages; Session +supported_languages; SendToUser/SendToGameChannel
+language. Docs (ARCHITECTURE/FUNCTIONAL/_ru/READMEs) + PLAN updated; stage marked done.

ui/src/screens/NewGame.svelte

@@ -6,12 +6,11 @@
   import { navigate } from '../lib/router.svelte';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import type { AccountRef, Variant } from '../lib/model';
+  import { availableVariants } from '../lib/variants';
 
-  const variants: { id: Variant; label: MessageKey }[] = [
-    { id: 'english', label: 'new.english' },
-    { id: 'russian', label: 'new.russian' },
-    { id: 'erudit', label: 'new.erudit' },
-  ];
+  // The offered variants are gated by the languages the sign-in service supports
+  // (Stage 15); the auto-match list and the friend-invite picker both use this.
+  const variants = $derived(availableVariants(app.session?.supportedLanguages));
   const timeouts = [
     { secs: 300, key: 'time.minutes' as MessageKey, n: 5 },
     { secs: 1800, key: 'time.minutes' as MessageKey, n: 30 },