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/data/media.rs

@@ -15,16 +15,6 @@ use crate::{
     ui::{keep, section, set},
 };
 
-macro_rules! asset {
-    ($name:literal) => {
-        concat!(
-            env!("CARGO_MANIFEST_DIR"),
-            "/examples/gallery/assets/",
-            $name
-        )
-    };
-}
-
 const PICTURES: [(&str, &str); 6] = [
     (asset!("dunes-sun.jpg"), "Sunset over the dunes"),
     (asset!("atrium-stair.jpg"), "A stair that turns"),