T24d: Agent, previews

- BrowserPreview: the address with a lock and a spinner while it loads, the
  host's frame shown, and the frames so far along the foot as Tab stops;
  ScreenshotStream is that strip
- ComputerUseViewer: the host's frame, a ring where the agent points that
  rings again at each new point, and what it does there
- LivePreview: the host's page under its address, with reload
- ArtifactPanel: title, versions through chat::BranchNavigator, Preview and
  Source when it has both, copy and download; it fails loud with neither
- gallery: frames cut from the gallery's own pages; one asset! macro in
  main.rs replaces three copies

examples/gallery/pages/agent/mod.rs

@@ -1,4 +1,5 @@
 mod changes;
+mod previews;
 mod progress;
 mod tools;
 
@@ -11,7 +12,7 @@ pub const PAGE: Page = Page {
     number: 24,
     slug: "agent",
     title: "Agent",
-    summary: "An assistant at work: the tools it calls, what it asks before it acts, how far it has come and what it would change.",
+    summary: "An assistant at work: the tools it calls, what it asks before it acts, how far it has come, what it would change, what it sees and what it makes.",
     render,
     script: SCRIPT,
 };
@@ -39,6 +40,9 @@ const SCRIPT: &[Step] = &[
     Step::DownAt("agent-changes", 30.0, 51.0),
     Step::UpAt("agent-changes", 30.0, 51.0),
     Step::Wait(200),
+    Step::Click("agent-pointer-next"),
+    Step::Wait(400),
+    Step::Shot("pointer"),
 ];
 
 fn render(window: &mut Window, cx: &mut App) -> AnyElement {
@@ -48,5 +52,7 @@ fn render(window: &mut Window, cx: &mut App) -> AnyElement {
         .child(progress::steps(window, cx))
         .child(progress::agents(window, cx))
         .child(changes::changes_section(window, cx))
+        .child(previews::watching(window, cx))
+        .child(previews::artifacts(window, cx))
         .into_any_element()
 }