fix(syncthing): Persist Syncthing state

Previously, the device ID would be lost and consequently regenerated on boot.
This means that previous connections will recognise it as a new, different
device.

users/crumb/default.nix

@@ -18,6 +18,8 @@
       ./files.nix
     ];
 
+    xdg.enable = true;
+
     sydnix = {
       sops = {
         enable = true;