assistant: Polish /workflow and steps UI (#15936)

Fixes #15923
Release Notes:

- Assistant workflow steps can now be applied and reverted directly from
within the assistant panel.

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>

crates/ui/src/components/icon.rs

@@ -256,6 +256,7 @@ pub enum IconName {
     TextSearch,
     Trash,
     TriangleRight,
+    Undo,
     Update,
     WholeWord,
     XCircle,
@@ -419,6 +420,7 @@ impl IconName {
             IconName::Trash => "icons/trash.svg",
             IconName::TriangleRight => "icons/triangle_right.svg",
             IconName::Update => "icons/update.svg",
+            IconName::Undo => "icons/undo.svg",
             IconName::WholeWord => "icons/word_search.svg",
             IconName::XCircle => "icons/error.svg",
             IconName::ZedAssistant => "icons/zed_assistant.svg",