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/README.md

@@ -28,12 +28,9 @@ pnpm codegen      # regenerate src/gen from edge.proto + scrabble.fbs (dev-time)
 `GATEWAY_URL` overrides the dev proxy target; `VITE_GATEWAY_URL` sets the runtime
 gateway origin for a packaged (non-proxied) build. `VITE_TELEGRAM_BOT_ID`
 enables the "Link Telegram" web sign-in (the Login Widget) — inert until the site
-domain is registered with BotFather (`/setdomain`); `VITE_TELEGRAM_LINK_EN` /
-`VITE_TELEGRAM_LINK_RU` are the per-bot friend-invite Mini App links (full URL
-`https://t.me/<bot>/<app>`; the share picks the one matching the bot the player
-signed in through, falling back to `VITE_TELEGRAM_LINK`).
-`VITE_TELEGRAM_GAME_CHANNEL_NAME_EN` / `VITE_TELEGRAM_GAME_CHANNEL_NAME_RU`
-are the per-language "Play in Telegram" links shown on the landing page.
+domain is registered with BotFather (`/setdomain`); `VITE_TELEGRAM_LINK` is the
+friend-invite Mini App link for the single bot (full URL `https://t.me/<bot>/<app>`).
+`VITE_TELEGRAM_GAME_CHANNEL_NAME` is the "Play in Telegram" link shown on the landing page.
 
 The build has **two entries**: the game SPA (`index.html`, served at `/app/` and
 `/telegram/`) and a lightweight landing page (`landing.html`, served at `/`).