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/components/ExportDataModal.jsx

@@ -31,7 +31,11 @@ export default function ExportDataModal({
   const [rowDim, setRowDim] = useState('date');
   const [colDim, setColDim] = useState('subject');
   const [pinnedId, setPinnedId] = useState(null);
-  const [groupBy, setGroupBy] = useState(groupField ?? '__none__');
+  const [groupBy, setGroupBy] = useState(() => {
+    const valid = groupField === '__none__' || groupField === '__name__' || groupField === '__id__'
+      || subjectTemplate.some((f) => f.active && f.fieldId === groupField);
+    return valid ? groupField : '__none__';
+  });
 
   useEffect(() => {
     let alive = true;