CubeComming had to dig through app.log to learn WHY a file failed ("integrity
check failed: Duration mismatch ..."). The reason was already returned in the
singles/process `errors` array and carried on the queue entry — the window just
showed "Failed" with no detail.
Now each failed file's reason renders under its row (red, left-bordered, with a
title tooltip for the full text). Pure presentation of data already present; no
API change. Vite build clean.
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