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
312c1f1da3