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.

backend/internal/server/dto.go

@@ -16,10 +16,11 @@ import (
 
 // sessionResponse is the credential returned by every auth endpoint.
 type sessionResponse 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"`
 }
 
 // okResponse is a simple success acknowledgement.
@@ -159,10 +160,11 @@ type errorBody struct {
 // sessionResponseFor builds the credential payload for a minted session.
 func sessionResponseFor(token string, acc account.Account) sessionResponse {
 	return sessionResponse{
-		Token:       token,
-		UserID:      acc.ID.String(),
-		IsGuest:     acc.IsGuest,
-		DisplayName: acc.DisplayName,
+		Token:           token,
+		UserID:          acc.ID.String(),
+		IsGuest:         acc.IsGuest,
+		DisplayName:     acc.DisplayName,
+		ServiceLanguage: acc.ServiceLanguage,
 	}
 }