style: Give notifications on lock screen the same width as in session

style: Give notifications on lock screen the same width as in session

There is no strong reason to use the same width as user list and prompt
for notifications, in particular as the smaller width can make the
notification's content appear more cramped.

Instead, give notifications the same width as the message list in
the session. Especially where we reuse the same component — namely
media messages — this ensures consistent presentation.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/work_items/9218
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4217>
(cherry picked from commit 6b52520d9d0c3e8d918dd87bb2a04dc3122755a5)