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

components/blog/PostCard.tsx

@@ -4,7 +4,7 @@ import Link from 'next/link';
 import { m } from 'motion/react';
 import type { PostMeta } from '@/lib/posts';
 
-export function PostCard({ slug, title, date, excerpt, tags = [], author, readingTime, index = 0 }: PostMeta & { index?: number }) {
+export function PostCard({ slug, title, date, excerpt, tags = [], author, readingTime, index = 0, coverImage }: PostMeta & { index?: number }) {
   return (
     <m.article
       initial={{ opacity: 0, y: 20 }}
@@ -14,6 +14,16 @@ export function PostCard({ slug, title, date, excerpt, tags = [], author, readin
       className="group relative scroll-mt-20 rounded-xl border border-border bg-canvas p-6 transition-all hover:border-border/80 hover:shadow-card"
     >
       <Link href={`/posts/${slug}/`} className="block">
+        {coverImage && (
+          <div className="mb-4 overflow-hidden rounded-lg">
+            <img
+              src={coverImage}
+              alt={title}
+              className="w-full h-40 object-cover"
+              loading="lazy"
+            />
+          </div>
+        )}
         <div className="flex items-center gap-3 text-xs text-ink-soft mb-3">
           <time className="font-mono">{date}</time>
           {author && <span className="text-ink-soft/70">·</span>}