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/components/contracts/GlCreateBookingForm.tsx

@@ -118,6 +118,7 @@ const blockNegative = (event: KeyboardEvent<HTMLInputElement>) => {
 
 interface UnitErrors {
   containerNumber?: string;
+  sealNumber?: string;
   vgmTons?: string;
 }
 
@@ -886,6 +887,9 @@ export default function GlCreateBookingForm() {
         } else if ((numberCounts.get(key) ?? 0) > 1) {
           errs.containerNumber = "Duplicate container number in this shipment.";
         }
+        if (u.sealNumber.trim() === "") {
+          errs.sealNumber = "Seal number is required.";
+        }
         const vgm = Number(u.vgmTons);
         if (u.vgmTons.trim() === "" || Number.isNaN(vgm) || vgm <= 0) {
           errs.vgmTons = "Enter a valid VGM.";
@@ -1185,7 +1189,7 @@ export default function GlCreateBookingForm() {
             : {}),
           units: l.units.map((u) => ({
             containerNumber: u.containerNumber.trim().toUpperCase(),
-            ...(u.sealNumber ? { sealNumber: u.sealNumber } : {}),
+            sealNumber: u.sealNumber.trim(),
             vgmTons: Number(u.vgmTons) || 0,
             // Per-container handling — the server rolls these into the line
             // counts and bills each surcharge on the ticked containers only.
@@ -1247,7 +1251,7 @@ export default function GlCreateBookingForm() {
           reeferQuantity: Number(l.reeferQuantity || 0) || undefined,
           units: l.units.map((u) => ({
             containerNumber: u.containerNumber.trim().toUpperCase(),
-            ...(u.sealNumber ? { sealNumber: u.sealNumber } : {}),
+            sealNumber: u.sealNumber.trim(),
             vgmTons: Number(u.vgmTons) || 0,
             isHazardous: Boolean(u.isHazardous),
             isReefer: Boolean(u.isReefer),
@@ -1857,7 +1861,7 @@ export default function GlCreateBookingForm() {
                             Container number *
                           </Text>
                           <Text fz={12} fw={600} c="#10202F" style={{ flex: 1 }}>
-                            Seal number
+                            Seal number *
                           </Text>
                           <Text fz={12} fw={600} c="#10202F" style={{ flex: 1 }}>
                             VGM (tons) *
@@ -1901,8 +1905,13 @@ export default function GlCreateBookingForm() {
                             style={{ flex: 1 }}
                           />
                           <TextInput
-                            placeholder="Optional"
+                            placeholder="e.g. SL-0099231"
                             value={unit.sealNumber}
+                            error={
+                              showErrors
+                                ? unitErrors[lineIdx]?.[unitIdx]?.sealNumber
+                                : undefined
+                            }
                             onChange={(e) =>
                               patchUnit(lineIdx, unitIdx, {
                                 sealNumber: e.currentTarget.value,