Mark items as dragging before updating inventory state to prevent
the spawner from destroying containers mid-drag. Move inventory
removal to drop time for lost items, and add guards to ignore
stale pointer events on inactive containers.
Replace manual ItemRenderer with InventoryItemSpawner and
GridBackgroundRenderer. Uses spawnEffect to automatically sync item
visuals with game state changes. Separates static grid rendering and
tracks dragged items to prevent spawner conflicts.
hypercross