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.

gateway/internal/backendclient/api.go

@@ -14,10 +14,11 @@ import (
 
 // SessionResp is the credential minted by an auth operation.
 type SessionResp struct {
-	Token       string `json:"token"`
-	UserID      string `json:"user_id"`
-	IsGuest     bool   `json:"is_guest"`
-	DisplayName string `json:"display_name"`
+	Token           string `json:"token"`
+	UserID          string `json:"user_id"`
+	IsGuest         bool   `json:"is_guest"`
+	DisplayName     string `json:"display_name"`
+	ServiceLanguage string `json:"service_language"`
 }
 
 // ProfileResp is an account's own profile.