style: update product page CSS for improved layout and flex behavior

public/themes/dist_static/static/css/product.css

@@ -440,14 +440,14 @@ body.is-p-subnav .site-header {
   display: flex;
   flex-wrap: nowrap;
   justify-content: center;
-  gap: 5rem;
+  gap: 5rem; /* 80px @1920 */
   border: 0;
   overflow-x: auto;
   -webkit-overflow-scrolling: touch;
   scrollbar-width: none;
 }
 .p-spec__tab {
-  flex: 1;
+  flex: 0 0 auto;
   display: flex;
   flex-direction: column;
   align-items: center;