Check if there is at least one editable calendar

[micz]

Check if there is at least one editable calendar when opening the new calendar event dialog. See #250.

[micz]

From https://github.com/micz/ThunderAI/issues/250#issuecomment-2776143588:

This fails with rev2 ("Error opening the calendar event dialog: No editable calendar found!"):

This doesn't fail:

The disabled calendar is a local calendar. The other two are network calendars (native caldav with different providers).