Adds a "Cancel backup" button on the backup progress screen (visible while
the backup runs). Cancelling requires TWO confirmations; on confirm it
stops the stepped-backup loop, best-effort cancels the record server-side
(ajax.cancelBackup), and returns to the dashboard WITHOUT continuing any
pending update. The button is hidden once the backup completes or fails.
Claude-Session: https://claude.ai/code/session_01WbGBN9VyRK61zczYWcCQ2i