Stability and performance fixes:

- Plain-text messages no longer cause a 15-second wait per message during
analysis. Scanning wide date ranges is now much faster.
- Fixed: those same messages were listed in the report as "attachments not
downloaded" even when they had no attachments at all.
- Fixed: two attachments from different messages that matched in name and
exact size were treated as duplicates, and the second one was not
downloaded. They are now distinguished by their destination folder.
- More robust parallel downloading: an isolated failure no longer interrupts
the remaining downloads in the same session.

- Fixed: in some cases, the same attachment could be counted or
downloaded more than once, showing a "downloaded attachments"
count higher than the files actually saved in the destination
folder.

- The TXT report generated at the end is now more detailed: it
distinguishes between files downloaded in the current session,
files that already existed from a previous download, and
attachments skipped as duplicates — including the subject,
sender, and date of each email to make manual verification
easier when needed.

- Attachments that could not be downloaded are now listed with
the subject, sender, and date of the corresponding email, to
make them easy to locate in Thunderbird.

- Reduced the wait time for newly added IMAP account
synchronization (from 60 to 15 seconds), avoiding unnecessary
long waits.

- Fixed an inconsistency in the final notification that could
show a total attachment count different from the one in the
report.