The mobile grid column was implicit (auto), so it sized to content — a long
workspace name (truncate = nowrap, wide intrinsic size) stretched the form
column to its max-w-lg (512px) and overflowed a 375px viewport by ~31px on the
workspaces list. Make the column an explicit minmax(0,1fr) track and give the
grid item min-w-0 so it stays within the viewport and the name truncates.
Found by a runtime horizontal-overflow check; adds MobileOverflowTest guarding
it across the layout variants (AuthLayout, default, full-width editor, detail).
