desktop: add oo7-daemon as the org.freedesktop.secrets provider

Without a secret-service implementation on the bus, libsecret clients
like flare fail at startup with 'The communication with libsecret
failed'. None of the desktop hosts had one wired up.

oo7-daemon is the matching pure-Rust implementation (same project as
the oo7 crate flare uses internally), without the GNOME plumbing that
gnome-keyring would drag in. Register the package's D-Bus service
file and systemd user unit, start the daemon at user login, and alias
the unit as dbus-org.freedesktop.secrets.service so D-Bus
auto-activation also resolves cleanly when the wantedBy start hasn't
fired yet.

Verified the toplevel build and that the resulting system carries the
oo7-daemon user unit, the dbus alias symlink, and the
default.target.wants entry.

modules/desktop-common.nix

@@ -19,6 +19,7 @@
     ./desktop-networkmanager.nix
     ./desktop-age-secrets.nix
     ./desktop-lanzaboote-agenix.nix
+    ./desktop-oo7-daemon.nix
 
     inputs.disko.nixosModules.disko