Reverts getColCount to the minimize-empty-cells heuristic from v1.68.0
after the aspect-ratio rewrite in 576856d63 broke 2×2 layout for 4
systems at common widget widths (240–359px).
Lowered MIN_CELL_WIDTH from 180 to 140 — a compromise between the
original 120 (v1.68.0) and the current 180, leaving room for the
DiskUsage component while keeping the 2×2 threshold at 280px.
Fixes#6385