[FIX] mail: correct breadcrumb when open discuss from chat window

Before this commit, when opening Discuss from chat window and
then accessing the settings page of a channel, the breadcrumb
showed "Unamed" as the name of the App instead of "Discuss".

This happens because the client action was not named when expanding
the chat window by opening Discuss app.

closes odoo/odoo#118941

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>