Each continuation batch of a multi-batch ZIM embed runs as a fresh
BullMQ job, so handle() ran the hardcoded `safeUpdateProgress(job, 5)`
even when the file was already 100k articles into a 600k-article ZIM.
The UI gauge briefly dropped to 5% before the per-batch onProgress
callback caught up to the true overall percentage, reading as a
backward jump every time a new batch started.
Compute initialPercent from batchOffset / totalArticles when available,
falling back to 5 for single-batch files (uploaded PDFs, txts) where
totalArticles isn't set. Capped at 99 to leave headroom for the 100%
final-batch marker.
Follow-up to PR #880 (which fixed the 0-100% scaling during a batch
but still had the initial-frame regression).
68c0a37cab