theme/calendar: Use em instead of px for calendar day sizes

The layout manager of the calendar/notifications popup is using the
calendar width to determine the width of the whole right area of the
calendar popup. When enabling large text, this means the cards used for
world clocks, weather etc. get larger, while the calender doesn't get
larger, leading to the cards getting clipped.

Make sure the calendar grows in size just as the cards do when large
text is enabled and define its size in em's instead of pixels.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5392

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2282>