[FIX] point_of_sale: control if a cashier is in localstorage

Before this commit, we could temporarily change cashier from A to B, and back to A seamlessly just by refreshing the page.
This could be a security issue for a cashier is held responsible for its sales.

This commit stores the cashier in localstorage, and with the right sets of methods, corrects the issue.

OPW 767827

closes #19207