Channel playlists (the collapsible sections on a channel page) now each carry the
standard watchlist button — Add to Watchlist / In Watchlist — so you can follow a
channel's playlist without pasting its link. The /youtube/playlists/<id> endpoint
flags each playlist with its follow state to hydrate the button; the button click
is handled before the row's expand toggle (stopPropagation), and adds via
followPlaylist — the same plumbing the search chip + watchlist use. Followed ones
then appear in the watchlist Channels tab alongside channels.
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