style(storybook): widen the story page inner to 80vw from 1920px

Below 1920px the inner keeps min(100%, 1280px) minus the gutters; from 1920px it
is 80vw (1536px at 1920, 2048px at 2560), centered, with no horizontal scroll.

src/components/story-page.css

@@ -18,6 +18,11 @@
   min-width: 0;
   margin-inline: auto;
 }
+/* Wide screens (2026-10-22): 80% of the viewport, i.e. 1536px at 1920px, growing beyond it (vw, so a scrollbar does not
+   shrink it). Below 1920px the rule above applies. */
+@media (min-width: 1920px) {
+  .sb-page > .sb-page-inner { width: 80vw; }
+}
 
 .sb-page .sb-page-header {
   padding-bottom: 24px;