Fix category column count regression

Fix category column count regression

T55130 redesigned category view to use dynamic CSS-based columns
instead of the old fixed 3-column layout. The design was updated
in I20b68dea8f, setting the column width to 25em.

However, on 1280px screen, this reduces the layout to just 2 columns,
as 3 columns do not fit: With the default 14px font, 25em translates
to 350px of column width, plus 1em of column-gap, therefore, 1078px
of space would be required for three columns. However, a 1280px
screen seems to have only 1038px left for content, so 40px are
missing.

By reducing the column size slightly to 24em, three columns are back.

Bug: T55130
Change-Id: I2dbecf809c54d786579bdf3826efbf85ad822c7c