feat(session): carry the bot service_language on the Session wire

Thread the Telegram bot's service language (en/ru) from the session mint response
through the gateway into the FlatBuffers Session, so the UI knows which bot the
player signed in through. handleTelegramAuth refreshes the account's service
language onto the response before minting (it was set after the fetched copy).
Empty for a non-Telegram login.

pkg/fbs/scrabble.fbs

@@ -123,6 +123,10 @@ table Session {
   is_guest:bool;
   display_name:string;
   supported_languages:[string];
+  // service_language is the language tag (en/ru) of the Telegram bot the session was
+  // minted through; empty for a non-Telegram login. The UI uses it to build a share
+  // link that points at the same bot.
+  service_language:string;
 }
 
 // Ack is a simple success acknowledgement (e.g. an email-code request).