Five fixed 296px columns overflowed 13" MacBooks (~1556px footprint vs
~1440 viewport), clipping the Done column. Columns now grow/shrink from
a zero basis, capped at --col (pixel-identical at >=1556px) and floored
at a new --col-min token (240px: five columns + gaps + board padding =
1276px, fitting 1280px viewports). Below the floor the existing
overflow-x:auto fallback is unchanged.
Sessions, Focus and the header are already fluid at 1280px — checked,
nothing to fix there.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>