Apply the same dimming strategy used in Prepare mode: when an element
is highlighted, reduce stroke and fill opacity of all other bboxes
so the focused element stands out clearly.
Extract bounding boxes from chunk doc_items provenance in the chunker,
propagate through domain/service/API layers, and render highlighted
bboxes on canvas when hovering a chunk card. Reset highlights on
mode and page changes to prevent stale visual state.
