[FIX] mail: proper hover feedback of message action list buttons

Follow-up of https://github.com/odoo/odoo/pull/90893



Improvements of message action list introduced a gap of clickable
part of button between button and the container of buttons.

This commit fixes the issue by removing this gap.

Also fixes an issue where the icon buttons were too small in mobile.

Task-2862366

closes odoo/odoo#92196

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>