feat(telegram): localized /start welcome with channel & chat follow links

The main bot answered /start with a single English line ("Tap to open Scrabble.").
Localize it: Russian or English by the sender's reported Telegram language
(Message.from.language_code, which the Bot API carries on the message itself — there is
no separate user-update event — English fallback), with the longer welcome copy and a
localized launch button ("Открыть «Эрудит»" / "Open “Erudite”").

The welcome links the game channel and the discussion chat by their public @username,
resolved once at startup from the configured TELEGRAM_GAME_CHANNEL_ID / TELEGRAM_CHAT_ID
via getChat and cached. A handle that is unset, private, or unreadable degrades to a
generic noun ("the channel" / "our chat") rather than a dangling "@", so the paragraph
always reads cleanly (the bot's info screen still lists the real links). Adds
GameChannelID to bot.Config (wired from the existing config) for the channel handle.

Tests: startText localization + handle embedding + per-slot generic fallback; handleStart
language selection; resolveWelcomeHandles. README updated.

platform/telegram/cmd/bot/main.go

@@ -72,6 +72,7 @@ func run(ctx context.Context, cfg config.BotConfig, logger *zap.Logger) error {
 		MiniAppURL:        cfg.MiniAppURL,
 		SendRatePerSecond: cfg.SendRatePerSecond,
 		ChatID:            cfg.ChatID,
+		GameChannelID:     cfg.GameChannelID,
 	}, logger)
 	if err != nil {
 		return err