[IMP] point_of_sale: Navigation as employee from frontend to backend.

We added a button for the front end closure to go back to the back end.
Before this, the only way to go back was to change the URL, now we can close the session with the frontend properly.

Also moved the css related to pos_hr in the good directory.
Task-id: 2032178

closes odoo/odoo#34948

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>