Add option to render the highlights on exported image

[qu1ck]

From #478

[andrewhofmann]

This would be really useful for assembly documentation workflows—I often want to export a snapshot showing which components are highlighted for a particular assembly step or review note, and currently have to screenshot manually. One thought: since the highlighting data is already in-memory, maybe a checkbox in the export dialog could toggle whether the highlight overlay gets composited into the exported image? That seems like it could be relatively contained compared to #478's broader export requests. Would love to see this added.