fix: restore deck group masonry columns

The card-size refactor set the group container's column-width to
min(20rem, 100%). column-width only accepts a <length>, not a percentage,
so the value was invalid and the layout collapsed to a single column.

Use a plain rem length (size.widthRem). Cards still clamp to the viewport
via their own min(width, 100%), and column-width naturally drops to one
column on narrow screens.

Author: cfbender

assets/react/src/pages/decks/deck-stack-grid.tsx

@@ -57,7 +57,7 @@ export function DeckGroupGrid({
     <div
       className="mx-auto gap-8"
       style={{
-        columnWidth: `min(${size.widthRem}rem, 100%)`,
+        columnWidth: `${size.widthRem}rem`,
         maxWidth: `calc(${columns} * ${size.widthRem}rem + ${Math.max(columns - 1, 0)} * 2rem)`,
       }}
     >