Roving tabindex over the card grid: arrows move (grid-aware by measured
column count), Home/End jump, Enter opens, Space selects, Delete removes
with the undo toast; focus rides the virtualizer window (scroll-to-row,
bounded retry focus). The container carries list semantics with a
translated label, and the transient upload queue stays a polite live
region. Selection state is conveyed by the per-card checkbox, which the
focus ring reveals for keyboard users.
Refs: document-vault plan W3-T3
