The active-downloads poll kept running in the background — the page-change event that
stops it only fires for video-side navigations, not when switching to the music side,
so /api/video/downloads/active was being hit forever. poll() now bails (and clears the
timer) whenever the Downloads page isn't on screen (data-side!=video or the subpage is
hidden). Also matched the music page's 2s active cadence (6s idle). Still HTTP polling,
same as the music downloads page (which polls /api/downloads/all every 2s — the app's
SocketIO is for other realtime, not the downloads list).
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