Remove the nested Pane from the assistant

Since we don't want tabs, I think it would be better to render the toolbar
for ourselves directly and handle switching between conversations.

Co-Authored-By: Julia Risley <julia@zed.dev>

crates/project_panel/src/project_panel.rs

@@ -153,6 +153,7 @@ pub fn init(cx: &mut AppContext) {
     );
 }
 
+#[derive(Debug)]
 pub enum Event {
     OpenedEntry {
         entry_id: ProjectEntryId,