fix(warehouses): add container size to standalone container-return form

Backend already accepted containerSize ('20'|'40') on
CreateEmptyContainerReturnDto and enforces one-40ft-or-two-20ft-per-wagon
via assertWagonLoad — the Standalone Return modal just never collected it.
Add a Container Type select and wire it into the submit payload.

apps/edr-freight-web/backoffice/src/pages/warehouses/ContainerReturnsPage.tsx

@@ -961,6 +961,7 @@ interface StandaloneReturnModalProps {
 
 function StandaloneReturnModal({ opened, onClose, onSubmit, loading }: StandaloneReturnModalProps) {
   const [containerNumber, setContainerNumber] = useState<string>("");
+  const [containerSize, setContainerSize] = useState<EmptyContainerSize | null>(null);
   const [returnedBy, setReturnedBy] = useState<"EDR" | "CUSTOMER" | null>(null);
   const [returnDate, setReturnDate] = useState<string>(new Date().toISOString().split("T")[0]);
   const [warehouse, setWarehouse] = useState<string | null>(null);
@@ -1021,6 +1022,7 @@ function StandaloneReturnModal({ opened, onClose, onSubmit, loading }: Standalon
           containers: [
             {
               containerNumber,
+              containerSize: containerSize ?? undefined,
               returnDate,
               warehouse: selectedWarehouse?.name || warehouse,
               yard: selectedYard?.name,
@@ -1034,6 +1036,7 @@ function StandaloneReturnModal({ opened, onClose, onSubmit, loading }: Standalon
     });
 
     setContainerNumber("");
+    setContainerSize(null);
     setReturnedBy(null);
     setReturnDate(new Date().toISOString().split("T")[0]);
     setWarehouse(null);
@@ -1071,6 +1074,17 @@ function StandaloneReturnModal({ opened, onClose, onSubmit, loading }: Standalon
           required
         />
 
+        <Select
+          label="Container Type"
+          placeholder="Select container size"
+          value={containerSize}
+          onChange={(val) => setContainerSize(val as EmptyContainerSize | null)}
+          data={[
+            { value: "20", label: "20 ft" },
+            { value: "40", label: "40 ft" },
+          ]}
+        />
+
         <Select
           label="Return Warehouse"
           placeholder="Select warehouse for container return"