agent: Add item to add custom MCP server in the panel's menu (#29091)

This is based on user feedback that the Agent Panel menu was only
linking to extensions as a way to add MCP servers while we also support
adding "custom" servers, too, which don't go through the extensions
flow.

Release Notes:

- N/A

crates/ui/src/components/modal.rs

@@ -249,10 +249,14 @@ impl ModalFooter {
 impl RenderOnce for ModalFooter {
     fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
         h_flex()
-            .flex_none()
             .w_full()
+            .mt_4()
             .p(DynamicSpacing::Base08.rems(cx))
-            .justify_between()
+            .flex_none()
+            .justify_end()
+            .gap_1()
+            .border_t_1()
+            .border_color(cx.theme().colors().border_variant)
             .child(div().when_some(self.start_slot, |this, start_slot| this.child(start_slot)))
             .child(div().when_some(self.end_slot, |this, end_slot| this.child(end_slot)))
     }