fix(export): default group-by to None when unset/stale; polish

Final code review of the configurable-data-export feature: ExperimentDetail
was forwarding its on-page groupField state (defaults to '__name__') into
ExportDataModal, leaking a clustering default into the default (untouched)
export. Now passes the raw saved localStorage value, defaulting to
'__none__'. The modal also now validates the incoming groupField against
'__none__'/'__name__'/'__id__'/active subjectTemplate fields and coerces to
'__none__' otherwise, guarding against stale/deleted subject_info fields.
Plus a stale module comment fix and optional-chaining consistency fix in
listSubjectMembers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/tests/ExportDataModal.test.jsx

@@ -22,6 +22,44 @@ beforeEach(() => {
   client.experimentsApi.getDailyStatuses.mockResolvedValue(statuses);
 });
 
+// Intercepts the Blob passed to URL.createObjectURL during handleExport so tests
+// can inspect the actual CSV text produced, rather than inferring it from the
+// <select>'s DOM value (which can be misleading — see the coercion test below).
+// Also stubs anchor .click() — jsdom treats a real click on an <a href="blob:...">
+// as a navigation attempt, which it doesn't implement; that throws asynchronously
+// (after the test has moved on) and can flakily fail an unrelated later test.
+function mockDownload() {
+  let blob = null;
+  const origCreate = global.URL.createObjectURL;
+  const origRevoke = global.URL.revokeObjectURL;
+  const origClick = window.HTMLAnchorElement.prototype.click;
+  global.URL.createObjectURL = jest.fn((b) => { blob = b; return 'blob:mock'; });
+  global.URL.revokeObjectURL = jest.fn();
+  window.HTMLAnchorElement.prototype.click = jest.fn();
+  return {
+    getBlob: () => blob,
+    // handleExport schedules `setTimeout(() => URL.revokeObjectURL(url), 0)`; flush
+    // that pending macrotask before unmocking, or it fires after restore() has put
+    // back the (non-existent, in jsdom) original revokeObjectURL and throws async —
+    // an error jest attributes to whatever test happens to run next.
+    restore: async () => {
+      await new Promise((r) => setTimeout(r, 0));
+      global.URL.createObjectURL = origCreate;
+      global.URL.revokeObjectURL = origRevoke;
+      window.HTMLAnchorElement.prototype.click = origClick;
+    },
+  };
+}
+
+function readBlobText(blob) {
+  return new Promise((resolve, reject) => {
+    const fr = new FileReader();
+    fr.onload = () => resolve(fr.result);
+    fr.onerror = reject;
+    fr.readAsText(blob);
+  });
+}
+
 function renderModal(props = {}) {
   return render(
     <ExportDataModal
@@ -61,4 +99,38 @@ describe('ExportDataModal', () => {
     fireEvent.change(rows, { target: { value: 'subject' } }); // collides with column=subject
     await waitFor(() => expect(screen.getByLabelText('Columns')).not.toHaveValue('subject'));
   });
+
+  it('defaults Group by to None when groupField is __none__, and the exported CSV has no Group row (Issue-1 regression guard: default export reproduces today\'s export)', async () => {
+    const dl = mockDownload();
+    renderModal({ groupField: '__none__' });
+    await screen.findByLabelText('Rows');
+    expect(screen.getByLabelText('Group by')).toHaveValue('__none__');
+    await waitFor(() => expect(screen.getByText(/1 date/i)).toBeInTheDocument());
+    fireEvent.click(screen.getByText('Export CSV'));
+    const text = await readBlobText(dl.getBlob());
+    expect(text).not.toMatch(/^Group,/m);
+    await dl.restore();
+  });
+
+  it('defaults Group by to the saved field when it is a valid active subject field', async () => {
+    renderModal({ groupField: 'grp' });
+    await screen.findByLabelText('Rows');
+    expect(screen.getByLabelText('Group by')).toHaveValue('grp');
+  });
+
+  it('coerces Group by to None when the saved field no longer exists in subjectTemplate, so the export carries no stray Group row (Issue-2 guard)', async () => {
+    // Note: asserting the <select>'s DOM .value here would be misleading — React's
+    // controlled <select> falls back to selecting the first <option> ("__none__")
+    // whenever the given value matches no option, regardless of whether the
+    // component actually validated/coerced its internal state. So we assert the
+    // real behavioral effect (the exported CSV) instead.
+    const dl = mockDownload();
+    renderModal({ groupField: 'ghost-field' });
+    await screen.findByLabelText('Rows');
+    await waitFor(() => expect(screen.getByText(/1 date/i)).toBeInTheDocument());
+    fireEvent.click(screen.getByText('Export CSV'));
+    const text = await readBlobText(dl.getBlob());
+    expect(text).not.toMatch(/^Group,/m);
+    await dl.restore();
+  });
 });