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/transcode/transcode_social.go

@@ -246,6 +246,10 @@ func invitationCancelHandler(backend *backendclient.Client) Handler {
 func profileUpdateHandler(backend *backendclient.Client) Handler {
 	return func(ctx context.Context, req Request) ([]byte, error) {
 		in := fb.GetRootAsUpdateProfileRequest(req.Payload, 0)
+		prefs := make([]string, in.VariantPreferencesLength())
+		for i := range prefs {
+			prefs[i] = string(in.VariantPreferences(i))
+		}
 		p := backendclient.ProfileResp{
 			DisplayName:            string(in.DisplayName()),
 			PreferredLanguage:      string(in.PreferredLanguage()),
@@ -255,6 +259,7 @@ func profileUpdateHandler(backend *backendclient.Client) Handler {
 			BlockChat:              in.BlockChat(),
 			BlockFriendRequests:    in.BlockFriendRequests(),
 			NotificationsInAppOnly: in.NotificationsInAppOnly(),
+			VariantPreferences:     prefs,
 		}
 		out, err := backend.UpdateProfile(ctx, req.UserID, p)
 		if err != nil {