The previous approach cloned the card DOM node and used removeChild to move it,
which meant the source node was gone on re-runs and a stale clone could persist
across history restores. Replace with: clear continueGrid, build a fresh <a>
element from the source card's attributes/innerHTML on every run, and hide
(not remove) the original in #books-grid so it is always available as a clean
source. Also scope the data-slug selector to #books-grid to avoid accidental
matches against the continue-reading cards themselves.
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