needs a license identifier

[ToyKeeper]

To clarify the usage of this code, service_worker.js needs a header stating what the license is. This is usually done in a standard format, such as:

// Always Right service_worker.js
// Copyright (C) 2025 Dietrich Ayala
// SPDX-License-Identifier: GPL-3.0-or-later

This goes at the top of the file.