download of a large folder only starts after the archive has been created

When downloading a folder using the archive download the web ui shows an activity visualization bar at the top after making the request to the server. Unfortunately, the download does not immediately start streaming but seems to hang until the archive has been fully assembled on the server side. This may cause a proxy in between to kill the connection. Furthermore, a user might be confused because nothing is happening (apart from the activity visualization bar at the top).

We should either start streaming immediately or show a notification that explains that the server is preparing the download ... but that should actually be async ... and a completely different API for downloads then what we currently have ... so ... we should just stream.

Trace the archive download request from the web UI to the server-side archive assembly and inspect where the response begins. Start by reproducing a large-folder download and observing when bytes are sent. Done means the download starts streaming before the archive is fully assembled, avoiding an idle connection while preserving the existing download behavior.

Written by the indexing model from the issue text.