feat(train-scheduling): load returned empties onto an export train

Empties had no way onto a departure: the return record could name a train
but nothing seated it on a wagon. Export schedules now expose a loading
action that packs selected returns onto free wagons at one 40ft or two 20ft
each, enforced both in the picker and in the API (existing empties on the
schedule count against their wagon).

Adds container_size, train_schedule_id and wagon_sequence_no to
freight.empty_container_returns.

apps/edr-freight-web/backoffice/src/pages/trainScheduling/TrainScheduleV2DetailPage.tsx

@@ -57,6 +57,7 @@ import { ContainerPlacementGrid } from "@/components/trainScheduling/ContainerPl
 import { FleetAvailabilitySummary } from "@/components/trainScheduling/FleetAvailabilitySummary";
 import { IntercityRideAlongPanel } from "@/components/trainScheduling/IntercityRideAlongPanel";
 import { LegCapacityPanel } from "@/components/trainScheduling/LegCapacityPanel";
+import { LoadEmptyContainersModal } from "@/components/trainScheduling/LoadEmptyContainersModal";
 import MergeScheduleTrainModal from "@/components/trainScheduling/MergeScheduleTrainModal";
 import ScheduleHistoryPanel from "@/components/trainScheduling/ScheduleHistoryPanel";
 // import { ImportLoadingConfirmationPanel } from "@/components/trainScheduling/ImportLoadingConfirmationPanel";
@@ -126,6 +127,7 @@ export default function TrainScheduleV2DetailPage() {
   const [dispatchConfirmOpen, setDispatchConfirmOpen] = useState(false);
   const [switchTarget, setSwitchTarget] = useState<EligibleContainerBooking | null>(null);
   const [visualization3DOpen, setVisualization3DOpen] = useState(false);
+  const [loadEmptiesOpen, setLoadEmptiesOpen] = useState(false);
   const autoPreviewedRef = useRef(false);
 
   const detailQuery = useQuery(
@@ -967,6 +969,20 @@ export default function TrainScheduleV2DetailPage() {
                 Merge
               </Button>
             ) : null}
+            {/* Empties ride an export departure back to Djibouti — offered only
+                while the train can still take load. */}
+            {schedule.direction === "EXPORT" &&
+            ["DRAFT", "SCHEDULED"].includes(schedule.status) ? (
+              <Button
+                variant="light"
+                color="edr-green"
+                size="compact-sm"
+                leftSection={<ContainerIcon size={14} />}
+                onClick={() => setLoadEmptiesOpen(true)}
+              >
+                Load Empty Container
+              </Button>
+            ) : null}
             {(schedule.trainSet?.wagons?.length ?? 0) > 0 ? (
               <Button
                 variant="gradient"
@@ -1365,6 +1381,12 @@ export default function TrainScheduleV2DetailPage() {
         onSaved={() => void detailQuery.refetch()}
       />
 
+      <LoadEmptyContainersModal
+        opened={loadEmptiesOpen}
+        onClose={() => setLoadEmptiesOpen(false)}
+        schedule={schedule}
+      />
+
       <MergeScheduleTrainModal
         scheduleId={scheduleId ?? null}
         currentTrainId={schedule.trainSet?.trainId ?? null}