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/lib/variants.ts

@@ -0,0 +1,32 @@
+// Game variants offered on New Game, and the Stage 15 gating of that choice by the
+// languages the sign-in service supports. Kept out of the .svelte screen so the
+// gating is unit-testable (the project's node-env Vitest layer).
+
+import type { MessageKey } from './i18n/index.svelte';
+import type { Variant } from './model';
+
+// VariantOption is a selectable game variant with its i18n label key.
+export interface VariantOption {
+  id: Variant;
+  label: MessageKey;
+}
+
+// ALL_VARIANTS lists every variant in display order.
+export const ALL_VARIANTS: VariantOption[] = [
+  { id: 'english', label: 'new.english' },
+  { id: 'russian', label: 'new.russian' },
+  { id: 'erudit', label: 'new.erudit' },
+];
+
+// VARIANT_LANGUAGE maps each variant to its game language. en -> English;
+// ru -> Russian + Эрудит.
+export const VARIANT_LANGUAGE: Record<Variant, 'en' | 'ru'> = { english: 'en', russian: 'ru', erudit: 'ru' };
+
+// availableVariants gates ALL_VARIANTS by the session's supported languages. An empty
+// or absent set is ungated (a web/legacy session without a declared set), returning
+// every variant.
+export function availableVariants(supportedLanguages: string[] | undefined): VariantOption[] {
+  const langs = supportedLanguages ?? [];
+  if (langs.length === 0) return ALL_VARIANTS;
+  return ALL_VARIANTS.filter((v) => langs.includes(VARIANT_LANGUAGE[v.id]));
+}