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/transcode/encode.go

@@ -36,6 +36,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	token := b.CreateString(s.Token)
 	uid := b.CreateString(s.UserID)
 	name := b.CreateString(s.DisplayName)
+	svcLang := b.CreateString(s.ServiceLanguage)
 	langs := buildSupportedLanguagesVector(b, supportedLangs)
 	fb.SessionStart(b)
 	fb.SessionAddToken(b, token)
@@ -43,6 +44,7 @@ func encodeSession(s backendclient.SessionResp, supportedLangs []string) []byte
 	fb.SessionAddIsGuest(b, s.IsGuest)
 	fb.SessionAddDisplayName(b, name)
 	fb.SessionAddSupportedLanguages(b, langs)
+	fb.SessionAddServiceLanguage(b, svcLang)
 	b.Finish(fb.SessionEnd(b))
 	return b.FinishedBytes()
 }

gateway/internal/transcode/transcode_telegram_test.go

@@ -43,7 +43,7 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 			t.Errorf("unexpected path %q", r.URL.Path)
 		}
 		_ = json.NewDecoder(r.Body).Decode(&gotBody)
-		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван"}`))
+		_, _ = w.Write([]byte(`{"token":"tok-tg","user_id":"u-tg","is_guest":false,"display_name":"Иван","service_language":"ru"}`))
 	})
 	defer cleanup()
 
@@ -67,6 +67,11 @@ func TestTelegramAuthForwardsSeedFields(t *testing.T) {
 	if got := sessionLanguages(sess); len(got) != 1 || got[0] != "ru" {
 		t.Errorf("session supported_languages = %v, want [ru]", got)
 	}
+	// The bot's service language rides the Session so the UI can build a share link to
+	// the same bot.
+	if got := string(sess.ServiceLanguage()); got != "ru" {
+		t.Errorf("session service_language = %q, want ru", got)
+	}
 	// The validated launch fields are forwarded so the backend can seed a new account;
 	// service_language is recorded to route the account's later out-of-app push.
 	if gotBody["external_id"] != "42" || gotBody["language_code"] != "ru" || gotBody["first_name"] != "Иван" || gotBody["service_language"] != "ru" {