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

TASKS.md

@@ -279,3 +279,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T24b | 2 | PASS | |
 | T24c | 1 | FAIL | at 280px the path vanished and Accept all drew outside the box; the diff's fold rows took presses but no Tab |
 | T24c | 2 | PASS | |
+| T24d | 1 | FAIL | at 280px eight thumbnails ran past the browser card; the artifact panel lost its title and its controls ran out of the box |