agent_ui: Improve the modal to add LLM providers (#42983)

Closes https://github.com/zed-industries/zed/issues/42807

This PR makes the modal to add LLM providers a bit better to interact
with:

1. Added a scrollbar
2. Made the inputs navigable with tab
3. Added some responsiveness to ensure it resizes on shorter windows


https://github.com/user-attachments/assets/758ea5f0-6bcc-4a2b-87ea-114982f37caf

Release Notes:

- agent: Improved the modal to add LLM providers by making it responsive
and keyboard navigable.

crates/ui/src/components/modal.rs

@@ -77,6 +77,7 @@ impl RenderOnce for Modal {
                     .w_full()
                     .flex_1()
                     .gap(DynamicSpacing::Base08.rems(cx))
+                    .when(self.footer.is_some(), |this| this.pb_4())
                     .when_some(
                         self.container_scroll_handler,
                         |this, container_scroll_handle| {
@@ -276,7 +277,6 @@ impl RenderOnce for ModalFooter {
     fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
         h_flex()
             .w_full()
-            .mt_4()
             .p(DynamicSpacing::Base08.rems(cx))
             .flex_none()
             .justify_between()