Stage 15: dual Telegram bots & language-gated variants

Service-agnostic refinement of the owner's idea: the sign-in service returns a
set of supported game languages with the user identity, and the lobby gates the
New Game variant choice by it (en -> English; ru -> Russian + Эрудит).

- Connector hosts two bots in one container (one per service language, each its
  own token + game channel; the same telegram_id spans both). ValidateInitData
  tries each token and returns the validating bot's service_language +
  supported_languages. Per-language config (TELEGRAM_BOT_TOKEN_EN/_RU, channels).
- supported_languages rides the Session (fbs, session-scoped, not persisted); the
  UI offers only the matching variants on New Game — gating only the START of a
  new game (auto-match + friend invite), not accept/open/play; backend does not
  enforce.
- service_language persisted (accounts.service_language, migration 00010, written
  every login, last-login-wins) and routes the user-facing Notify push back
  through the right bot (push-target coalesces with preferred_language).
- Admin SendToUser/SendToGameChannel gain an operator-chosen language selector in
  the console (unrelated to ValidateInitData).
- Non-Telegram logins carry the gateway default set
  (GATEWAY_DEFAULT_SUPPORTED_LANGUAGES, all variants).

Wire (committed regen): ValidateInitDataResponse +service_language
+supported_languages; Session +supported_languages; SendToUser/SendToGameChannel
+language. Docs (ARCHITECTURE/FUNCTIONAL/_ru/READMEs) + PLAN updated; stage marked done.

ui/src/lib/codec.ts

@@ -274,9 +274,22 @@ function decodeChatMsg(m: fb.ChatMessage): ChatMessage {
   };
 }
 
+// sessionFromTable projects a FlatBuffers Session table (a root or a nested one) to
+// the Session model, including the supported-languages set the UI gates variants by.
+function sessionFromTable(t: fb.Session): Session {
+  const supportedLanguages: string[] = [];
+  for (let i = 0; i < t.supportedLanguagesLength(); i++) supportedLanguages.push(s(t.supportedLanguages(i)));
+  return {
+    token: s(t.token()),
+    userId: s(t.userId()),
+    isGuest: t.isGuest(),
+    displayName: s(t.displayName()),
+    supportedLanguages,
+  };
+}
+
 export function decodeSession(buf: Uint8Array): Session {
-  const t = fb.Session.getRootAsSession(new ByteBuffer(buf));
-  return { token: s(t.token()), userId: s(t.userId()), isGuest: t.isGuest(), displayName: s(t.displayName()) };
+  return sessionFromTable(fb.Session.getRootAsSession(new ByteBuffer(buf)));
 }
 
 export function decodeProfile(buf: Uint8Array): Profile {
@@ -525,9 +538,7 @@ export function decodeLinkResult(buf: Uint8Array): LinkResult {
     secondaryDisplayName: s(r.secondaryDisplayName()),
     secondaryGames: r.secondaryGames(),
     secondaryFriends: r.secondaryFriends(),
-    session: sess
-      ? { token: s(sess.token()), userId: s(sess.userId()), isGuest: sess.isGuest(), displayName: s(sess.displayName()) }
-      : null,
+    session: sess ? sessionFromTable(sess) : null,
   };
 }