Make coauthor changes clearer for assigned posts
Assigned authors can see who is credited on a post and understand which author changes their role allows.
Teams working on assigned posts can now see the current author list and the limits of each role without guessing what a save button will do.
- See the full author list: assigned authors can view the names and avatars of the other people credited on their post.
- Understand available changes: options that are outside the current role are visibly disabled with an explanation, while assigned authors can add or remove themselves on posts assigned to them.
- Get useful feedback: failed author updates and refreshes now show an actionable message instead of failing silently.