webui: version/date download URL

[mr-tz]

Currently, there's one hard-coded URL for the download of the offline capa Explorer web tool. A new version would use the same URL but provide a different file.

Ideally, we version the tool (by version or data/timestamp) so that other tools can rely on a scheme (e.g. FLARE VM where downloaded files are compared to an expected hash value for data integrity).