style(runbackup): black backdrop on the full-screen backup screen
Set html/body background to #000 so the full-screen pre-update / Backup
Now progress screen runs on a black backdrop (the card + Atum-coloured
progress bar sit on top).
Claude-Session: https://claude.ai/code/session_01WbGBN9VyRK61zczYWcCQ2i
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