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.

docs/UI_DESIGN.md

@@ -313,10 +313,10 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
 - **Friend code**: the issued code sits next to a 📋 copy control; tapping the code or
   the icon copies it. **Share via Telegram** wraps the code in a `t.me/<bot>/<app>?startapp=`
   deep-link and opens Telegram's native share-to-chat sheet (Web Share / clipboard fallback
-  outside Telegram); the link and its caption are for the **same bot the player signed in
-  through** (its service language). Redeeming your **own** invite shows a friendly note, not an
+  outside Telegram); the link and its caption are for the **single bot**. Redeeming your **own**
+  invite shows a friendly note, not an
   error; opening an **outdated** invite link (a used or expired single-use code) lands in the
-  lobby with a calm modal pointing at the right bot (`@<username>`), not a red error on the
+  lobby with a calm modal pointing at the bot (`@<username>`), not a red error on the
   Friends tab. Flex text inputs carry `min-width:0` so they shrink instead of overflowing in
   Safari.
 - **History / GCG**: the in-game slide-down history lays each move out in a per-seat grid