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

src/agent/tests/mod.rs

@@ -4,6 +4,7 @@ use gpui::{
 };
 
 mod changes;
+mod previews;
 
 use super::{AgentProgress, Permission, PermissionPrompt, ToolApprovalDialog};
 use crate::{forms, primitives::FocusNext, theme::Theme};