feat: cover image display + date formatting + footer spacing

- PostCard: render coverImage above meta/content with lazy loading
- Post detail: render coverImage below title with lazy loading
- Format dates via toLocaleDateString (e.g. Jan 15, 2025)
- Footer top margin: mt-20 → mt-16

app/posts/[slug]/page.tsx

@@ -44,7 +44,7 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
         <div className="grid grid-cols-1 lg:grid-cols-[1fr_200px] gap-8">
           <article>
             <header className="mb-12">
-              <time className="font-mono text-sm text-ink-soft">{post.date}</time>
+              <time className="font-mono text-sm text-ink-soft">{new Date(post.date).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' })}</time>
               <h1 className="heading-xl text-ink mt-3 mb-2">
                 {post.title}
               </h1>
@@ -52,6 +52,14 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
                 {post.author && <span>by {post.author}</span>}
                 <span>{post.readingTime} min read</span>
               </div>
+              {post.coverImage && (
+                <img
+                  src={post.coverImage}
+                  alt={post.title}
+                  className="w-full h-64 object-cover rounded-xl my-6"
+                  loading="lazy"
+                />
+              )}
               {post.tags.length > 0 && (
                 <div className="flex flex-wrap gap-2 mt-4">
                   {post.tags.map((tag) => (