Fix editor & assistant panel toolbar height mismatch (#15552)

This PR ensures that the toolbar within the editor and the assistant
panel have the same height. I also pushed in some other tiny design
(spacing, really) tweaks. There's still a mismatch between the editor
toolbar and the assistant panel's history tab, but I'll try to tackle
this separately.

---

Release Notes:

- N/A

crates/workspace/src/toolbar.rs

@@ -105,6 +105,7 @@ impl Render for Toolbar {
             .bg(cx.theme().colors().toolbar_background)
             .child(
                 h_flex()
+                    .min_h(rems_from_px(24.))
                     .justify_between()
                     .gap(Spacing::Large.rems(cx))
                     .when(has_left_items, |this| {