Yeah this unfortunately doesn't exist the way I'd like it either. I just simply won't use an ebook reader if it's just running in the browser. It really needs a dedicated app. There were two awesome ebook readers, one called marvin3, the other I don't know the name. They both kinda stopped dead end, and since they were unfortunately closed source, even if the community wanted to, we can't just pick up where the author abandoned the work. one of my dreams is to make a completely floss ebook ecosystem. A central collection/library that can serve to however many clients. A client that's a full application for ios and android, as well as computers/macs. Also a client for appletv and other tv ecosystems so you can just lay on your couch or on your bed and read without having to actually hold the thing in your hand. Eating popcorn while reading a spy thriller the likes of john lecarre and you will be converted. All of which are as featureful and customizable as marvin. I'd rather not do it myself but it's just kinda one of those things that really needs to be done!

I'd really love to see the client app have customizable skins/chrome. you could have an elegant woody, a futuristic white tablet, an aged leather bound tome. Or just no theme at all. You could customize the context menu for when you highlight a phrase or a word. Want different actions available on longpress, go for it.

I have written so many pages of features and ideation but haven't even come up with a name, let a lone started any coding whatsoever 😅

Of course interop would be a central concern. Serve you library to any client that accepts it's standard api. Add a plugin to the server so it works on whatever client you like! Fully open source so if the project is abandoned, it can be forked an given new life.

...Yeah I really have dreamt about something like this for a looooong time