describeDownloadError condenses a postprocessing failure to one line for
the queue/notification, discarding the rest of yt-dlp's stderr. The
persisted error log now also keeps the fuller non-noise transcript
(stderrDetail) as an optional `detail` field, and the Diagnostics card
gets a collapsible "Show details" expander per row so a failure like
"Postprocessing: Conversion failed!" can be inspected -- and copied
into a bug report -- without re-running the download from a terminal.
yt-dlp collapses an ffmpeg postprocessing failure to a bare
"Postprocessing: Conversion failed!" with no indication of which step
died, and without --verbose the underlying ffmpeg reason is never
emitted at all. describeDownloadError() appends the last non-noise
trailing lines (e.g. the [ModifyChapters] SponsorBlock re-encode) so
the error log and completion notification point at the actual cause
instead of a dead end.
debont80