Interesting, it doesn’t seem to contain anything that would be needed here.
I would have assumed it would check for Akonadi to be present.
Will have to check the clock widget code again.
Yes, a command line client to KDE’s calendar system.
I would just be surprised if it were available on Bazzite but maybe it is.
No, the “Add” button becomes visible if either KOrganizer or Merkuro Calendar are installed. Or more precisely if either org.kde.korganizer.desktop
or org.kde.merkuro.calendar
are found.