One can use hypothes.is on mobile Safari with the bookmarklet. I didn't really expect it was going to work, but it does.
If someone can explain how to use this for Firefox/Android and/or EinkBro (Android), My Dumb Friend would appreciate the pointer. Otherwise, hypothes.is appears largely to be a server-side implementation --- that is, included into page source, not something trivially applied client-side.
* Bookmark a different site.
* Go through the bookmark management to edit that bookmark
* Replace the URL with the javascript:// in the bookmarklet here (https://maya.land/technicalities/hypothesis/dark-mode/) for dark mode, or from the ordinary Hypothes.is bookmarklet otherwise
* Navigate to a page you want to annotate
* Select that bookmarklet
And it works as well as in Firefox desktop, which is to say, it irritates me to have to log in all the time, but otherwise peachy.
I'm not totally clear on what you mean about the implementation being "server-side". There is a server-side implementation to save the annotations. The client and annotation format is also complicated enough that I can't really imagine hacking a different version together for non-browser-based use. But at the same time, it's simple enough to apply where the JS can run that I feel like "trivial" isn't a terrible descriptor.
(See also, for desktop: a user script to get around people's irritating browser policies that interfere with the client https://maya.land/user-scripts/hypothesis/ )