feat(warehouses): backoffice UI for container stack positions

Surfaces the physical stack/slot model in the staff app.

- ZoneLayoutModal: stacks drawn level by level with occupancy colours,
  configured-vs-built-vs-occupied counts, and stack create/delete plus
  block/reserve/free on empty levels
- SlotPicker in the store and move modals, offering only the next
  fillable level of each stack so the form cannot suggest a position
  the API will refuse
- move modal warns when a container is buried, lists the blockers, and
  disables the action instead of firing a 409
- fix: move() now asserts accessibility server-side, matching release —
  both are exits from a stack

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

@@ -12,7 +12,7 @@ import {
   Tabs,
   Text,
 } from '@mantine/core';
-import { ArrowLeft, Boxes, LayoutGrid, Package, Pencil, Plus, Trash2 } from 'lucide-react';
+import { ArrowLeft, Boxes, Layers, LayoutGrid, Package, Pencil, Plus, Trash2 } from 'lucide-react';
 import { useMutation, useQuery } from '@tanstack/react-query';
 import { DataTable, type ColumnDef } from '@edr/ui-common';
 
@@ -25,6 +25,7 @@ import {
   WarehouseTypeBadge,
   ZoneContentsModal,
   type ZoneRef,
+  ZoneLayoutModal,
   ZoneOccupancyHeatmap,
   formatCapacity,
   humanizeEnum,
@@ -63,6 +64,7 @@ export default function WarehouseDetailPage() {
   const [editingZone, setEditingZone] = useState<WarehouseZone | null>(null);
   const [selectedYardId, setSelectedYardId] = useState<string | null>(null);
   const [contentsZone, setContentsZone] = useState<ZoneRef | null>(null);
+  const [layoutZone, setLayoutZone] = useState<ZoneRef | null>(null);
 
   const zonesQuery = useQuery(
     api.warehouses.listZones.queryOptions({
@@ -203,6 +205,14 @@ export default function WarehouseDetailPage() {
         meta: { headerClassName: 'text-right', cellClassName: 'text-right' },
         cell: ({ row }) => (
           <Group gap={4} justify="flex-end" wrap="nowrap" onClick={(e) => e.stopPropagation()}>
+            <ActionIcon
+              variant="subtle"
+              color="gray"
+              title="Stack layout"
+              onClick={() => setLayoutZone(row.original)}
+            >
+              <Layers size={16} />
+            </ActionIcon>
             <ActionIcon
               variant="subtle"
               color="gray"
@@ -425,6 +435,12 @@ export default function WarehouseDetailPage() {
           yard={editingYard}
         />
       )}
+      <ZoneLayoutModal
+        opened={Boolean(layoutZone)}
+        onClose={() => setLayoutZone(null)}
+        zone={layoutZone}
+      />
+
       <ZoneContentsModal
         opened={Boolean(contentsZone)}
         onClose={() => setContentsZone(null)}