Fixes the 'cancelled but still shows running' stuck bug and adds real depth:
- classify_state now distinguishes 'cancelled' from 'failed'.
- Monitor is robust to slskd forgetting a transfer: if it's gone, it first tries to
complete from the FILE on disk (survives the music 'Clean Completed Downloads'
auto-clear), else counts misses and fails the row after ~8 polls instead of hanging
on 'downloading' forever. Cancelled transfers → cancelled status.
- POST /downloads/cancel (slskd DELETE transfer + mark cancelled) and /downloads/retry
(re-grab the same release). get_video_download(id) added; clear includes cancelled.
process_download stays pure (fs/slskd injected); 15 tests, ruff + guard clean.
Phase B (page: tabs/queue/history/cancel+retry buttons) next.
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