onWikipediaDownloadComplete was deleting every file whose name starts
with `wikipedia_en_`, treating distinct corpora (simple, medicine,
wikivoyage, climate_change, etc.) as competing versions of the same
selection slot. Whichever wiki finished second silently wiped the
other from disk.
Match by filename stem instead — strip the trailing `_YYYY-MM(-DD).zim`
date suffix and only delete files with the same stem as the new
download. Different release dates of the same variant still get cleaned
up; distinct variants are preserved.
Extracted the predicate to `app/utils/zim_filename.ts` so the boundary
is covered by unit tests (8 cases incl. the #884 repro scenario).
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