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

tasks/ch21-30.md

@@ -185,11 +185,11 @@
 
 ## T24d Agent · Previews
 
-- [ ] BrowserPreview / ScreenshotStream — host frames
-- [ ] ComputerUseViewer — host frames
-- [ ] ArtifactPanel
-- [ ] ArtifactVersionSwitcher
-- [ ] LivePreview — host content
+- [x] BrowserPreview / ScreenshotStream — host frames → `agent::BrowserPreview`, the frames so far along its foot
+- [x] ComputerUseViewer — host frames
+- [x] ArtifactPanel
+- [x] ArtifactVersionSwitcher → `chat::BranchNavigator`, in `agent::ArtifactPanel`
+- [x] LivePreview — host content
 
 ## T24e Agent · Environment