[FR]: Use collapsing header for topics screen

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the problem

The header in the topics screen should be a collapsing toolbar and not a list item. This would also improve the accuracy of the scroll bars.

### Describe the solution

Use a collapsing toolbar so the header item is not part of the list.

### Additional context

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Start by locating the topics screen and its current header list item, then review how the screen's scrolling and scroll bars are implemented. Done means the header uses a collapsing toolbar rather than being part of the list, with accurate scroll bar behavior.

Written by the indexing model from the issue text.