assistant2: Add headers to chat messages (#11191)

This PR adds headers to the chat messages in the new assistant panel.

Adapted from the work on the `assistant-chat-ui` branch.

Release Notes:

- N/A

crates/assistant2/src/ui/chat_message_header.rs

@@ -0,0 +1,57 @@
+use client::User;
+use std::sync::Arc;
+use ui::{prelude::*, Avatar};
+
+pub enum UserOrAssistant {
+    User(Option<Arc<User>>),
+    Assistant,
+}
+
+#[derive(IntoElement)]
+pub struct ChatMessageHeader {
+    player: UserOrAssistant,
+    contexts: Vec<()>,
+}
+
+impl ChatMessageHeader {
+    pub fn new(player: UserOrAssistant) -> Self {
+        Self {
+            player,
+            contexts: Vec::new(),
+        }
+    }
+}
+
+impl RenderOnce for ChatMessageHeader {
+    fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
+        let (username, avatar_uri) = match self.player {
+            UserOrAssistant::Assistant => (
+                "Assistant".into(),
+                Some("https://zed.dev/assistant_avatar.png".into()),
+            ),
+            UserOrAssistant::User(Some(user)) => {
+                (user.github_login.clone(), Some(user.avatar_uri.clone()))
+            }
+            UserOrAssistant::User(None) => ("You".into(), None),
+        };
+
+        h_flex()
+            .justify_between()
+            .child(
+                h_flex()
+                    .gap_3()
+                    .map(|this| {
+                        let avatar_size = rems(20.0 / 16.0);
+                        if let Some(avatar_uri) = avatar_uri {
+                            this.child(Avatar::new(avatar_uri).size(avatar_size))
+                        } else {
+                            this.child(div().size(avatar_size))
+                        }
+                    })
+                    .child(Label::new(username).color(Color::Default)),
+            )
+            .child(div().when(!self.contexts.is_empty(), |this| {
+                this.child(Label::new(self.contexts.len().to_string()).color(Color::Muted))
+            }))
+    }
+}