assistant panel: Animate assistant label if message is pending (#16152)

This adds a pulsating effect to the assistant header in case the message
is pending.

The pulsating effect is capped between 0.2 and 1.0 and I tried (with the
help of Claude) to give it a "breathing" effect, since I found the
normal bounce a bit too much.

Also opted for setting the `alpha` on the `LabelLike` things, vs.
overwriting the color, since I think that's cleaner instead of exposing
the color and mutating that.


https://github.com/user-attachments/assets/4a94a1c5-8dc7-4c40-b30f-d92d112db7b5


Release Notes:

- N/A

crates/ui/src/components/label/highlighted_label.rs

@@ -53,6 +53,11 @@ impl LabelCommon for HighlightedLabel {
         self.base = self.base.italic(italic);
         self
     }
+
+    fn alpha(mut self, alpha: f32) -> Self {
+        self.base = self.base.alpha(alpha);
+        self
+    }
 }
 
 pub fn highlight_ranges(