feat(telegram,game): single bot + per-user variant preferences

Collapse the two per-language Telegram bots into one unified bot and
replace language-based variant gating with explicit per-user variant
preferences.

- Telegram: one bot; drop service_language and the supported_languages
  set everywhere (DB, account, auth, FlatBuffers Session wire, gateway,
  connector proto). The single bot renders chat and out-of-app push in
  the recipient's preferred_language; remove the game-language push
  routing override (notify Intent.Language / push Event.language).
- Preferences: new accounts.variant_preferences (text[], DB default
  {erudit_ru}, CHECK non-empty + subset of the three variants). Gates
  the New Game picker, vs-AI and the friend invitation the player
  creates, enforced server-side (HTTP 400 otherwise); an invited friend
  may still accept any variant. Edited on the Settings screen; variants
  are Erudit-first everywhere.
- Admin: drop the per-bot language selectors (broadcast / send-to-user)
  and the feedback channel_lang column/field.
- Env/CI: collapse TELEGRAM_BOT_TOKEN_{EN,RU}, TELEGRAM_GAME_CHANNEL_ID_{EN,RU},
  VITE_TELEGRAM_LINK{,_EN,_RU} and VITE_TELEGRAM_GAME_CHANNEL_NAME_{EN,RU}
  to single unsuffixed names; drop GATEWAY_DEFAULT_SUPPORTED_LANGUAGES.
- Docs updated (ARCHITECTURE, FUNCTIONAL + _ru, platform/telegram, gateway,
  backend, ui, UI_DESIGN, PRERELEASE).

The migration squash is deferred to a follow-up PR.

ui/src/screens/NewGame.svelte

@@ -18,9 +18,9 @@
   // The auto-match move clock (mirrors backend game.DefaultTurnTimeout = 24h).
   const AUTO_MATCH_HOURS = 24;
 
-  // The offered variants are gated by the languages the sign-in service supports;
-  // the auto-match list and the friend-invite picker both use this.
-  const variants = $derived(availableVariants(app.session?.supportedLanguages));
+  // The offered variants are gated by the player's profile preferences (the variants
+  // they enabled in Settings); the auto-match list and the friend-invite picker both use this.
+  const variants = $derived(availableVariants(app.profile?.variantPreferences));
   // "Multiple words per turn" off is the single-word rule; it is offered for Russian games
   // only (English is always standard and shows no toggle). Shared by both flows.
   let multipleWords = $state(false);