fix(stacks): show correct status when update service has failed

systemctl status exits non-zero for failed services (exit 3), not
only missing ones (exit 4), causing the || warn branch to always fire
after a failed run. Gate the "not installed" warning on whether the
unit file exists instead.

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

stacks.sh

@@ -508,8 +508,11 @@ EOF
       ;;
 
     status)
-      systemctl status stacks-update.timer stacks-update.service --no-pager 2>/dev/null \
-        || warn "Update service not installed"
+      if systemctl cat stacks-update.timer &>/dev/null; then
+        systemctl status stacks-update.timer stacks-update.service --no-pager 2>/dev/null || true
+      else
+        warn "Update service not installed"
+      fi
       ;;
 
     logs)