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.

gateway/internal/connector/client.go

@@ -27,18 +27,13 @@ var ErrInvalidInitData = errors.New("connector: invalid telegram init data")
 // rejects the Login Widget data (a gRPC InvalidArgument).
 var ErrInvalidLoginWidget = errors.New("connector: invalid telegram login widget data")
 
-// User is a validated Mini App identity. ServiceLanguage is the validating bot's
-// language tag (en/ru), persisted to route the user's out-of-app push back through
-// the right bot; SupportedLanguages is that bot's set of offered game languages,
-// which the UI gates the New Game variant choice by. Both are empty for a Login
-// Widget validation (it carries no bot language).
+// User is a validated Mini App identity. LanguageCode seeds a brand-new account's
+// preferred (interface) language; it is empty for a Login Widget validation.
 type User struct {
-	ExternalID         string
-	Username           string
-	FirstName          string
-	LanguageCode       string
-	ServiceLanguage    string
-	SupportedLanguages []string
+	ExternalID   string
+	Username     string
+	FirstName    string
+	LanguageCode string
 }
 
 // Client wraps the connector's Telegram gRPC service.
@@ -73,12 +68,10 @@ func (c *Client) ValidateInitData(ctx context.Context, initData string) (User, e
 		return User{}, err
 	}
 	return User{
-		ExternalID:         resp.GetExternalId(),
-		Username:           resp.GetUsername(),
-		FirstName:          resp.GetFirstName(),
-		LanguageCode:       resp.GetLanguageCode(),
-		ServiceLanguage:    resp.GetServiceLanguage(),
-		SupportedLanguages: resp.GetSupportedLanguages(),
+		ExternalID:   resp.GetExternalId(),
+		Username:     resp.GetUsername(),
+		FirstName:    resp.GetFirstName(),
+		LanguageCode: resp.GetLanguageCode(),
 	}, nil
 }