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 #5392 (closed)