gnome-integration: add Users-panel .desktop overlay and README

Adds the data file shipped by the optional authforge-gnome-integration
package: a /usr/share/applications/.desktop overlay tagged with
X-GNOME-Settings-Panel=user-accounts so gnome-control-center surfaces a
"Configure security…" launcher inside each user's detail view. The
README documents why a .desktop overlay was picked over a JS extension
(cross-version stability, packaging simplicity) and how to verify the
integration works in GNOME 46/47.

gnome-integration/io.dangerousthings.AuthForge.UsersPanel.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Name=Configure security…
+GenericName=Authentication keys & MFA
+Comment=Manage U2F/FIDO2 keys and authentication policy for this account
+Exec=authforge
+Icon=io.dangerousthings.AuthForge
+Terminal=false
+Type=Application
+NoDisplay=true
+X-GNOME-Settings-Panel=user-accounts
+X-GNOME-Single-Window=true
+OnlyShowIn=GNOME;
+Keywords=mfa;u2f;fido2;passkey;totp;security;