T24d round 1 fix

- BrowserPreview's frame strip scrolls sideways, so every frame stays in
  reach in a narrow card
- ArtifactPanel's header wraps: the title keeps label_width, the controls
  ride in one group that wraps inside the box
- tests: a_narrow_strip_scrolls_to_its_last_frame,
  a_narrow_panel_wraps_its_header
- AGENTS: everything works at 280px

src/agent/previews.rs

@@ -126,6 +126,8 @@ impl RenderOnce for BrowserPreview {
         let shown = self.shown;
         let strip = (self.frames.len() > 1).then(|| {
             div()
+                .id((self.id.clone(), "strip"))
+                .overflow_x_scroll()
                 .flex()
                 .gap_1p5()
                 .p_2()