install.sh now sets KGet's remembered "New Download" folder
(kgetrc Internal/LastDirectory) to the XDG downloads directory if it
isn't already configured, since KGet's own out-of-the-box default was
Documents. Falls back correctly even when xdg-user-dir has no
user-dirs.dirs config to read (it silently returns bare $HOME
otherwise).
Also add a GitHub Actions workflow that packages extension/ into a zip
and publishes a GitHub Release with generated notes whenever a
v*.*.* tag is pushed.
