[IMP] web: Calendar: mobile UX improvements

The following tweaks have been applied on mobile devices:

 - remove left and right arrow in control panel
 - enable swipe navigation (instead of left/right arrows)
 - replace 'Today' button by a calendar icon
 - remove static text 'Attendees:' in events
 - shorten header content in week mode to prevent them from
   overlapping
 - ensure that the calendar fits in the screen

Also introduce a mobile test suite for the calendar view.

Task #31449