Improve EVI hosting

note I: we could provide hashsums to [integrity protect](https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity) the files. 

note II: we could use unpkg or the like to pubish the scripts/styles.

note III: add a section in self-hosting on how to download + host the files

_Originally posted by @simne7 in https://github.com/identinet/check/pull/4#discussion_r2159323845_
            