Fix loading state in PO details modal

Fixed the loading prop name from 'isLoading' to 'loading' in UnifiedPurchaseOrderModal
to properly display loading state while fetching data from backend. This ensures users
see a loading indicator instead of an empty modal during data fetch.

frontend/src/components/domain/procurement/UnifiedPurchaseOrderModal.tsx

@@ -760,7 +760,7 @@ export const UnifiedPurchaseOrderModal: React.FC<UnifiedPurchaseOrderModalProps>
         }) : undefined}
         sections={sections}
         actions={buildActions()}
-        isLoading={isLoading}
+        loading={isLoading}
         size="lg"
         // Enable edit mode via standard Edit button (only for pending approval)
         onEdit={po?.status === 'pending_approval' ? () => setMode('edit') : undefined}