Videos: 2-per-row default, featured opt-in to wide layout

Reverted the global youtube wide treatment per user direction.
Videos default to span 6 (pair layout) at 600px+ again. Items with
`featured: true` in JSON get a new `card--wide` class that opts
them into the same 78%-centered + vertical-breathing layout
portfolio uses — meant to promote a hero video without making
every video imposing.

assets/js/app.js

@@ -60,8 +60,9 @@
   }
 
   function renderYouTube(it) {
+    const wide = it.featured ? " card--wide" : "";
     return `
-      <div class="card card--youtube reveal">
+      <div class="card card--youtube${wide} reveal">
         <div class="yt" role="button" tabindex="0" aria-label="Play: ${esc(it.title)}" data-yt="${esc(it.id)}" style="background-image:url('${esc(ytThumb(it.id))}')">
           <div class="yt__play" aria-hidden="true">
             <svg viewBox="0 0 24 24"><path d="M8 5v14l11-7z" fill="currentColor"/></svg>