agent2: Add new "new thread" selector in the toolbar (#36133)

Release Notes:

- N/A

crates/zed/src/zed/component_preview.rs

@@ -761,7 +761,7 @@ impl Render for ComponentPreview {
                         )
                         .track_scroll(self.nav_scroll_handle.clone())
                         .p_2p5()
-                        .w(px(229.))
+                        .w(px(231.)) // Matches perfectly with the size of the "Component Preview" tab, if that's the first one in the pane
                         .h_full()
                         .flex_1(),
                     )