Document component licenses and add SPDX identifiers #605

The root LICENSE contains GPLv2, but sensors.js and values.js carry BSD-3-Clause notices, and the flag icons have a separate MIT license. Mixed licensing is not a problem, but these distinctions are easy to miss when packaging the project.

Could the README include a licensing section that identifies the project’s default license and the exceptions? It would also help to clarify whether the other icons fall under the project license. The existing icon credits identify authors and sources, but do not explain the applicable licenses.

Please consider adding SPDX license identifiers to source files and equivalent metadata for assets. This would make the mixed licensing explicit and easier for packagers and automated tools to interpret.