Comment viewer (web UI): the flat tree gains search (highlight + keep
ancestor context), sort (threaded / top / newest / oldest), per-thread
collapse/expand with collapse-all/expand-all, an OP badge for the
uploader, and a "new since last visit" highlight + count (localStorage
per-video timestamp). API now returns each comment's `timestamp` and
`author_is_uploader`.
SponsorBlock: was a hardcoded `--sponsorblock-mark all`. Now a real
setting (off / mark / remove) with a per-channel override, threaded
through the full five touchpoints — config `[backup].sponsorblock_mode`
(default "mark", preserving prior behavior), DownloadOptions override,
a downloader resolver (apply_sponsorblock), and both UIs (egui Settings
+ channel dialog; web Settings modal + channel dialog + SettingsPayload).
Integration tests extended: settings round-trip asserts the sponsorblock
default + persistence; channel-options round-trip asserts the override.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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