Click any video card in Music Videos tab to download. Flow:
1. Search primary metadata source for clean artist/title
2. Fall back to YouTube title parsing if no match
3. Download video via yt-dlp (best quality MP4)
4. Save to configured Music Videos folder as Artist/Title-video.mp4
UI shows circular progress ring on the thumbnail during download,
green checkmark on completion, red X on error (clickable to retry).
Cards are non-interactive while downloading.
Backend: /api/music-video/download and /api/music-video/status endpoints
YouTube client: download_music_video() method keeps video format
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