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/README.md

@@ -38,10 +38,17 @@ Telegram identity to an account from a browser. Both map a rejection to gRPC
   operator-chosen for broadcasts) with a Mini App launch button and sends it. It replies
   with an `Ack` per command (`delivered` mirrors the former connector semantics —
   false when the kind is not rendered out-of-app or the user never started the bot).
-- **Bot chat.** `/start <payload>` (and the chat menu button) reply with a Mini App
-  launch button; a deep-link payload routes the launch to a game / invitation / friend
-  code. This is **self-contained** — the bot never calls back into the game, so `/start`
-  onboarding works even when the game is down.
+- **Bot chat.** `/start <payload>` (and the chat menu button) reply with a localized
+  welcome and a Mini App launch button; a deep-link payload routes the launch to a game /
+  invitation / friend code. The welcome is **Russian or English** by the sender's reported
+  Telegram language (`Message.from.language_code`, which the Bot API carries on the message
+  itself — no separate user-update event — English fallback) and links the game channel and
+  discussion chat by their public `@username`, **resolved once at startup** from
+  `TELEGRAM_GAME_CHANNEL_ID` / `TELEGRAM_CHAT_ID` via `getChat` (a chat that is unset,
+  private, or unreadable degrades that link to a generic noun — "the channel" / "our
+  chat" — rather than a dangling "@"). This is otherwise **self-contained**
+  — the bot never calls back into the game, so `/start` onboarding works even when the game
+  is down.
 - **Moderated-chat gating.** When `TELEGRAM_CHAT_ID` names a channel's linked discussion
   group, the bot gates who may write there. The group **allows sending by default** (a
   human setting) and the bot only **restricts** — Telegram intersects the chat default with