Fix: youtube cards rendered at 0 width when constrained

YouTube cards only have absolutely-positioned children
(.yt__play, .yt__title), so their intrinsic width is 0. With
max-width:78% + justify-self:center but no `width`, the cards
collapsed to nothing — explaining the empty Videos section.
Portfolio worked because its caption has text content giving the
card an intrinsic width.

Add width:100% to the constrained rule so max-width has a definite
basis to cap against. Both portfolio and youtube cards now render
at 78% of their grid track on 700px+.

assets/css/styles.css

@@ -286,6 +286,7 @@ main > .section:first-child { margin-top: var(--space-3xl); }
 @media (min-width: 700px) {
   .grid > .card--portfolio,
   .grid > .card--youtube {
+    width: 100%;
     max-width: 78%;
     justify-self: center;
   }