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;

frontend/src/lib/dataExport.js

@@ -1,4 +1,6 @@
-// Pure helpers for exporting daily-parameter data as a date×subject CSV matrix.
+// Pure helpers for exporting daily-parameter data as a CSV matrix. buildMatrix is
+// dimension-agnostic: rows and columns are assignable to Date/Subject/Parameter,
+// with the leftover dimension pinned to a single value.
 // No React, no I/O — mirrors the crossSubjectChart.js pure-helper pattern.
 
 // Read a single parameter's raw value from one daily status.
@@ -112,7 +114,7 @@ export function listSubjectMembers(animals, groupField) {
   const members = list.map((a) => {
     const name = a?.animal_name ?? '';
     const label = nameCounts.get(name) > 1 ? `${name} (${a?.animal_id_string ?? ''})` : name;
-    return { id: a.id, label, animalId: a.id, group: subjectGroupValue(a, groupField) };
+    return { id: a?.id, label, animalId: a?.id, group: subjectGroupValue(a, groupField) };
   });
   const grouped = !!groupField && groupField !== '__none__';
   members.sort((x, y) => {

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>