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/dataExport.test.js

@@ -136,6 +136,25 @@ describe('buildMatrix', () => {
       { statuses: gStatuses, animals: gAnimals, dailyTemplate });
     expect(m.columns.map((c) => c.label)).not.toContain('X');
   });
+
+  it('neither axis is Subject: drops a blank date row and a blank parameter column together', () => {
+    // Subject is pinned (rowDim=date, colDim=parameter). Both drop-blank filters engage:
+    // 07-02 has no data at all (row dropped), and the custom field X never has data (column dropped).
+    const dailyTemplate = [{ fieldId: 'c-x', key: 'x', label: 'X', builtin: false, active: true }];
+    const statuses = [
+      { animal_id: 'a1', date: '2026-07-01T00:00:00.000Z', analysis_summary: { total: 5, success_rate: 0.5 } },
+      { animal_id: 'a1', date: '2026-07-02T00:00:00.000Z', analysis_summary: null },
+    ];
+    const subjectMember = { id: 'a1', label: 'Alpha', animalId: 'a1' };
+    const m = buildMatrix(
+      { rowDim: 'date', colDim: 'parameter', pinnedMember: subjectMember, groupField: '__none__' },
+      { statuses, animals: gAnimals, dailyTemplate },
+    );
+    expect(m.groupAxis).toBeNull();
+    expect(m.rows.map((r) => r.member.label)).toEqual(['2026-07-01']);
+    expect(m.columns.map((c) => c.label)).not.toContain('X');
+    expect(m.columns.map((c) => c.label)).toEqual(['Total attempts', 'Success rate']);
+  });
 });
 
 import { toCSV, csvFilename } from '../src/lib/dataExport';