[FIX] point_of_sale: fix customer name overflow

To Reproduce
============
- start a session on pos
- on customer button create new one with a long name
- go back to pos dashboard, the name of the chosen customer goes outside of the button

Solution
========
a style was added to the button so that the long string is divided on multiple lines

opw-3074503

closes odoo/odoo#106144

Signed-off-by: Masereel Pierre <pim@odoo.com>