agent: Add ability to change the API base URL for OpenAI via the UI (#32979)

The `api_url` setting is one that most providers already support and can
be changed via the `settings.json`. We're adding the ability to change
it via the UI for OpenAI specifically so it can be more easily connected
to v0.

Release Notes:

- agent: Added ability to change the API base URL for OpenAI via the UI

---------

Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>

crates/ui_input/src/ui_input.rs

@@ -138,18 +138,18 @@ impl Render for SingleLineInput {
             .when_some(self.label.clone(), |this, label| {
                 this.child(
                     Label::new(label)
-                        .size(LabelSize::Default)
+                        .size(LabelSize::Small)
                         .color(if self.disabled {
                             Color::Disabled
                         } else {
-                            Color::Muted
+                            Color::Default
                         }),
                 )
             })
             .child(
                 h_flex()
                     .px_2()
-                    .py_1()
+                    .py_1p5()
                     .bg(style.background_color)
                     .text_color(style.text_color)
                     .rounded_md()