firefox-fourier: ship .desktop launcher (pkgrel=7)

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.

arch/firefox-fourier/firefox-fourier.desktop

@@ -0,0 +1,22 @@
+[Desktop Entry]
+Version=1.0
+Type=Application
+Name=Firefox (Fourier — V4L2 HW decode)
+GenericName=Web Browser
+Comment=Browse the Web with V4L2 stateless HW video decode (RK3588/RK3566)
+Exec=/usr/bin/firefox-fourier %u
+Icon=/usr/lib/firefox-fourier/browser/chrome/icons/default/default128.png
+Terminal=false
+StartupNotify=true
+StartupWMClass=firefox-fourier
+Categories=Network;WebBrowser;
+MimeType=text/html;text/xml;application/xhtml+xml;application/vnd.mozilla.xul+xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;image/svg+xml;image/webp;image/avif;application/json;application/pdf;audio/flac;audio/ogg;audio/webm;video/ogg;video/webm;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/chrome;x-scheme-handler/mailto;
+Actions=new-window;new-private-window;
+
+[Desktop Action new-window]
+Name=Open a New Window
+Exec=/usr/bin/firefox-fourier --new-window %u
+
+[Desktop Action new-private-window]
+Name=Open a New Private Window
+Exec=/usr/bin/firefox-fourier --private-window %u