feat(system): enable smartd for drive health monitoring

modules/nixos/default.nix

@@ -135,6 +135,16 @@ in
     # `services.fwupd.enable = false`.
     services.fwupd.enable = lib.mkDefault true;
 
+    # Drive health monitoring (SMART).
+    services.smartd = {
+      enable = lib.mkDefault true;
+      notifications.x11.enable = lib.mkDefault true;
+      notifications.wall.enable = lib.mkDefault true;
+    };
+    # Explicitly enable systembus-notify to resolve a mkDefault conflict
+    # between earlyoom (true) and smartd (false).
+    services.systembus-notify.enable = true;
+
     networking.networkmanager.enable = lib.mkDefault true;
     # OpenVPN support for the VPN menu's import/connect flow (nomarchy-vpn).
     # WireGuard needs no plugin (NetworkManager imports wg .conf natively);