Add GUI crate skeleton with libadwaita placeholder (build deferred until libgtk-4-dev/libadwaita-1-dev installed)

gui/data/io.dangerousthings.UbuntuFido.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=Authentication
+GenericName=Security Keys & MFA
+Comment=Manage U2F/FIDO2 keys and authentication policy
+Exec=ubuntu-fido
+Icon=io.dangerousthings.UbuntuFido
+Terminal=false
+Type=Application
+Categories=Settings;Security;
+Keywords=mfa;u2f;fido2;passkey;totp;security;
+StartupNotify=true