images/generate-thumbnails/progress is becoming POST-only, which the
browser EventSource cannot request. Both thumbnail progress consumers in
the gallery now use SseStream.connect(url, {method: 'POST'}) exactly as
the autotag streams do. SseStream gains addEventListener/removeEventListener
for named event: frames (tested) so the status, start, progress,
file_error, complete and error listeners stay as they were, and an onclose
guard rejects when the stream ends without a terminal frame.
Vito Sansevero
b792da53ba