feat: add slider preview thumbnails (#935)

packages/sandbox/app/shared/html/mux-storyboard.ts

@@ -0,0 +1,9 @@
+import { getMuxAssetId } from '../mux';
+import type { SourceId } from '../sources';
+
+export function renderMuxStoryboard(source: SourceId): string {
+  const id = getMuxAssetId(source);
+  return id
+    ? `<track kind="metadata" label="thumbnails" src="https://image.mux.com/${id}/storyboard.vtt" default />`
+    : '';
+}