feat(Auth): Unify shared PAM sync across greeter & lockscreen

- Add a neutral `dms auth sync` command and reuse the shared auth flow from:
- Settings auth toggle auto-apply
- `dms greeter sync`
- `dms greeter install`
- greeter auth cleanup paths

- Rework lockscreen PAM so DMS builds /etc/pam.d/dankshell from the system login stack, but removes fingerprint and U2F from that password path. Keep /etc/pam.d/dankshell-u2f separate.

- Preserve custom PAM files in place to avoid adding duplicate greeter auth when the distro already provides it, and keep NixOS on the non-writing path.

distro/nix/greeter.nix

@@ -139,6 +139,13 @@ in
         '';
       }
     ];
+    # DMS currently relies on /etc/pam.d/login for lock screen password auth on NixOS.
+    # Declare security.pam.services.dankshell only if you want to override that runtime fallback.
+    # U2F and fingerprint are handled separately by DMS — do not add pam_u2f or pam_fprintd here.
+    # security.pam.services.dankshell = {
+    #   # Example: add faillock
+    #   faillock.enable = true;
+    # };
     services.greetd = {
       enable = lib.mkDefault true;
       settings.default_session.command = lib.mkDefault (lib.getExe greeterScript);