fix(stasis): give the desktop plan its suspend step back

The desktop plan was built from the ac: blocks before it got its own, so a
machine without a battery did suspend after idle. Adding dpms and lock_screen
under default: dropped that -- the desktop stopped sleeping.

Timeout matches the ac: plan: 1800s after the blank, 36 min idle.

defaults/xdg/stasis/stasis.rune

@@ -75,12 +75,9 @@ default:
   # the action blocks living in ac:/battery: alone, a desktop ran a plan of
   # disabled steps: no DPMS, no lock, "waiting for idle" forever.
   #
-  # Deliberately only two steps:
-  #   brightness -- brightnessctl drives an internal panel; external monitors
-  #                 need DDC/CI, which it does not speak. /sys/class/backlight
-  #                 is empty on a desktop, so the step would be a no-op.
-  #   suspend    -- suspending a desktop is a separate decision, not part of
-  #                 idle screen handling.
+  # Only brightness is left out: brightnessctl drives an internal panel, and
+  # external monitors need DDC/CI, which it does not speak. /sys/class/backlight
+  # is empty on a desktop, so the step would be a no-op.
   lock_screen:
     timeout 300           # 5 min idle
     command "swaylock"
@@ -92,6 +89,11 @@ default:
     resume_command "niri msg action power-on-monitors"
   end
 
+  suspend:
+    timeout 1800          # 30 min after the blank, so 36 min idle
+    command "systemctl suspend"
+  end
+
   # AC power plan (relaxed)
   ac:
     brightness: