Enhance API error handling and response structure; update app name retrieval method

api/auth/login.php

@@ -18,6 +18,7 @@ if (!isset($input["username"]) || !isset($input["password"])) {
         "error" => "missing_fields",
         "error_text" => "Some fields are missing"
     ], true);
+    return;
 }
 
 $username = $input["username"];