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/src/pages/ExperimentDetail.jsx

@@ -417,7 +417,7 @@ export default function ExperimentDetail() {
           dailyTemplate={dailyTemplate}
           animals={animals}
           subjectTemplate={subjectTemplate}
-          groupField={groupField}
+          groupField={localStorage.getItem(`exp-subject-group-${id}`) ?? '__none__'}
           onClose={() => setShowExport(false)}
         />
       </Modal>