Stock firefox.desktop disappears when our 'provides=firefox' replaces stock
firefox-arch, so installing firefox-fourier left the user without a Plasma/GNOME
start-menu entry. Add firefox-fourier.desktop with Categories=Network;WebBrowser;
(routes under Internet on KDE Plasma 6), MIME types for the usual web schemes,
new-window / private-window actions, and the 128px icon from the package's
internal browser/chrome/icons tree.
Reported on ohm (PineTab2 / Plasma 6) — manual /usr/share/applications shim
proved the file works; this commits it to the recipe so future installs Just Work.
