Improve streaming vs. downloading UX

1. Many users get confused that streaming also downloads the file.
1. Some users would prefer to stream only and not download.

We need to both make this clearer and add options to allow people to automatically delete content after streaming.

Begin by tracing the existing streaming and download flow and the UI that explains it; the issue names no files or tests. Clarify whether streaming should avoid persistence and define the automatic-delete option, then document the expected UX and verify both behaviors.

Written by the indexing model from the issue text.