Add proper vertical margins (12px) to the SearchEntry widget in the
contacts view to improve spacing and visual consistency. This matches
the existing horizontal margins and follows GNOME design guidelines.
This change improves the visual appearance by preventing the search bar
from appearing cramped, especially when using high-contrast themes.
Closes: https://gitlab.gnome.org/GNOME/calls/-/issues/672
Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/762>
