add basic systemd service monitoring (#1153)

Co-authored-by: Shelby Tucker <shelby.tucker@gmail.com>

agent/systemd_nonlinux.go

@@ -0,0 +1,28 @@
+//go:build !linux
+
+package agent
+
+import (
+	"errors"
+
+	"github.com/henrygd/beszel/internal/entities/systemd"
+)
+
+// systemdManager manages the collection of systemd service statistics.
+type systemdManager struct {
+	hasFreshStats bool
+}
+
+// newSystemdManager creates a new systemdManager.
+func newSystemdManager() (*systemdManager, error) {
+	return &systemdManager{}, nil
+}
+
+// getServiceStats returns nil for non-linux systems.
+func (sm *systemdManager) getServiceStats(conn any, refresh bool) []*systemd.Service {
+	return nil
+}
+
+func (sm *systemdManager) getServiceDetails(string) (systemd.ServiceDetails, error) {
+	return nil, errors.New("systemd manager unavailable")
+}