collab: Refine call buttons design (#35007)

Making icons consistent, adjusting spacing, and moving the "Leave Call"
button to be the very last, which makes more sense to me than the
"Share" button being the last. Sharing your project is still part of the
call, so in the left edge of the button strip is where, conceptually,
the option to end the call should be, I think!

Release Notes:

- N/A

crates/ui/src/components/button/split_button.rs

@@ -12,6 +12,7 @@ use super::ButtonLike;
 pub enum SplitButtonStyle {
     Filled,
     Outlined,
+    Transparent,
 }
 
 /// /// A button with two parts: a primary action on the left and a secondary action on the right.
@@ -44,10 +45,17 @@ impl SplitButton {
 
 impl RenderOnce for SplitButton {
     fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
+        let is_filled_or_outlined = matches!(
+            self.style,
+            SplitButtonStyle::Filled | SplitButtonStyle::Outlined
+        );
+
         h_flex()
             .rounded_sm()
-            .border_1()
-            .border_color(cx.theme().colors().border.opacity(0.5))
+            .when(is_filled_or_outlined, |this| {
+                this.border_1()
+                    .border_color(cx.theme().colors().border.opacity(0.8))
+            })
             .child(div().flex_grow().child(self.left))
             .child(
                 div()