Set IMALISON_SESSION_TYPE env var for reliable X11/Wayland detection

systemd user environment persists across login sessions, so
XDG_SESSION_TYPE can be stale. Proactively set a custom variable
on each session start so ConditionEnvironment checks are reliable.

Use it for xsettingsd and random-background (X11-only services).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/home-manager.nix

@@ -56,6 +56,9 @@
     enable = true;
     preferStatusNotifierItems = true;
     importedVariables = [ "GDK_PIXBUF_ICON_LOADER" ];
+    profileExtra = ''
+      systemctl --user set-environment IMALISON_SESSION_TYPE=x11
+    '';
   };
 
   home.keyboard = null;