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.

backend/internal/server/handlers_admin_console.go

@@ -424,7 +424,6 @@ func (s *Server) consoleUserMessage(c *gin.Context) {
 	}
 	back := "/_gm/users/" + id.String()
 	text := trimForm(c, "text")
-	language := trimForm(c, "language")
 	switch {
 	case text == "":
 		s.renderConsoleMessage(c, "Nothing sent", "the message was empty", back)
@@ -436,14 +435,14 @@ func (s *Server) consoleUserMessage(c *gin.Context) {
 			s.renderConsoleMessage(c, "No Telegram", "this account has no Telegram identity", back)
 			return
 		}
-		delivered, err := s.connector.SendToUser(ctx, ext, text, language)
+		delivered, err := s.connector.SendToUser(ctx, ext, text)
 		if err != nil {
 			s.consoleError(c, err)
 			return
 		}
 		body := "message delivered"
 		if !delivered {
-			body = "not delivered (the user may not have started that bot)"
+			body = "not delivered (the user may not have started the bot)"
 		}
 		s.renderConsoleMessage(c, "Sent", body, back)
 	}
@@ -833,21 +832,20 @@ func (s *Server) consoleBroadcast(c *gin.Context) {
 // consolePostBroadcast posts an operator message to the connector's game channel.
 func (s *Server) consolePostBroadcast(c *gin.Context) {
 	text := trimForm(c, "text")
-	language := trimForm(c, "language")
 	switch {
 	case text == "":
 		s.renderConsoleMessage(c, "Nothing sent", "the message was empty", "/_gm/broadcast")
 	case s.connector == nil:
 		s.renderConsoleMessage(c, "Not configured", "the connector is not configured (set BACKEND_CONNECTOR_ADDR)", "/_gm/broadcast")
 	default:
-		delivered, err := s.connector.SendToGameChannel(c.Request.Context(), text, language)
+		delivered, err := s.connector.SendToGameChannel(c.Request.Context(), text)
 		if err != nil {
 			s.consoleError(c, err)
 			return
 		}
 		body := "posted to the game channel"
 		if !delivered {
-			body = "not delivered (that bot has no game channel configured)"
+			body = "not delivered (the bot has no game channel configured)"
 		}
 		s.renderConsoleMessage(c, "Broadcast", body, "/_gm/broadcast")
 	}