Smoother export downloads and carousel embeds

Export downloads now stay on the authenticated SlideDrift app path while the file is streamed from the export service. This improves PDF and PNG delivery for signed-in users without changing the normal export flow.

Public carousel examples in SlideDrift guides also have a cleaner embedded viewer, lighter page chrome, mobile-friendly controls, and a collapsible transcript so readers can swipe the example first and open the slide text only when they need it.