I often come across reading challenges that I don't want to join (for example because I usually don't do yearly reading challenges, or because I don't want to read books for all the prompts) but that I find interesting because they have nice prompts and I'd like to go back to the challenge and use it to find new books for my tbr down the line.

In that case, I'd love to be able to bookmark a challenge, instead of joining it, so I can access my bookmarks directly in the app and don't have to keep them in my browser (as I do now).