Refactor authentication: streamline cookie handling and enhance username retrieval in responses

api/auth/isauth.php

@@ -4,5 +4,6 @@ require "../api.php";
 $api = new gbAPI(true);
 
 $api->answer(200, [
-    "login" => true
+    "login" => true,
+    "username" => $api->username
 ]);