Add per-post banners (16:9 hero images) + banner-gen script; add banner to all posts

src/pages/posts/[...slug].astro

@@ -62,6 +62,11 @@ const switchHref = translation ? `/posts/${translation}/` : undefined;
   }}
 >
   <article>
+    {post.data.banner && (
+      <figure class="article-banner">
+        <img src={post.data.banner} alt="" loading="lazy" width="1600" height="900" />
+      </figure>
+    )}
     <header class="article-head">
       <h1>{post.data.title}</h1>
       <div class="meta">