Confusing sidebar panel collapsing

The sidebar icon in the black sidebar _closes_ the panel, whereas the icon in the chat panel makes it _full width_. I think it's confusing that they do different actions. I expected the sidebar icon in the chat panel to close it — I'm more likely wanting to close the chat than expand it full width. If I want to make it bigger I can just drag the panel to be bigger. Related, I wish I could close the chat panel easily when I don't need it.

Start by locating the sidebar and chat-panel UI entry points, then inspect how each panel icon currently behaves. Compare the existing close and full-width actions before deciding which controls can be reused. Done means the chat panel can be closed easily and its icon behavior matches the intended sidebar interaction.

Written by the indexing model from the issue text.