feat: update Zen browser version to 1.12.3b; change default browser to Firefox in dconf and default apps configuration

config/home-manager/workstation/dconf.nix

@@ -56,7 +56,7 @@
       # add more by listing them with `dconf read /org/gnome/shell/favorite-apps | sed "s/,//g; s/'/\"/g"` then copy pasting the output here
       "org/gnome/shell" = {
         favorite-apps = [
-          "zen.desktop"
+          "firefox.desktop"
           "nemo.desktop"
           "com.spotify.Client.desktop"
           "signal-desktop.desktop"