ui: fix problem download

packages/hydrooj/package.json

@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "2.39.20",
+    "version": "2.39.21",
     "bin": "bin/hydrooj.js",
     "main": "src/loader",
     "module": "src/loader",

packages/hydrooj/src/handler/problem.ts

@@ -45,6 +45,7 @@ registerValue('Problem', [
     ['domainId', 'String!'],
     ['docId', 'Int!'],
     ['docType', 'Int!'],
+    ['pid', 'String'],
     ['title', 'String!'],
     ['content', 'String!'],
     ['config', 'String!'],