ui: 支持自定义导出文件名

packages/ui-default/pages/problem_detail.page.jsx

@@ -104,7 +104,7 @@ const page = new NamedPage(['problem_detail', 'contest_detail_problem', 'homewor
   const extender = new ProblemPageExtender();
 
   async function handleClickDownloadProblem() {
-    await downloadProblemSet([UiContext.problemNumId]);
+    await downloadProblemSet([UiContext.problemNumId], UiContext.pdoc.title);
   }
 
   async function scratchpadFadeIn() {

packages/ui-default/pages/problem_main.page.js

@@ -202,9 +202,12 @@ async function handleOperation(operation) {
   }
 }
 
-async function handleDownload() {
+async function handleDownload(ev) {
+  let name = 'Export';
+  // eslint-disable-next-line no-alert
+  if (ev.shiftKey) name = prompt('Filename:', name);
   const pids = ensureAndGetSelectedPids();
-  await downloadProblemSet(pids);
+  await downloadProblemSet(pids, name);
 }
 
 const page = new NamedPage(['problem_main', 'problem_category'], () => {