Fixed the download manager progress popup showing "X / 0 files" instead of the real total. The pending-item count is now taken after the queue is populated, so the denominator reflects the actual number of papers being downloaded. The download manager now also surfaces an explicit alert when there is nothing to download (e.g., all papers already downloaded) or when queueing fails for every selected research session, instead of silently completing with "0 / 0 files".
