The 3MF parser sums prediction + weight across every plate (#1593) so the
archive card can headline the whole project — correct for the card, wrong
for the completion notification of a single plate. The queue UI already
re-reads the 3MF per-plate at print_queue.py:272-285; mirror that for the
notification path so Discord / Pushover / email show the plate's actual
duration and grams instead of the project sum. Helper fails open on every
error path so a missing or corrupt 3MF can't block the notification.
