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/layout/Footer.tsx

@@ -1,6 +1,6 @@
 export function Footer() {
   return (
-    <footer className="border-t border-border mt-20 py-8 text-center">
+    <footer className="border-t border-border mt-16 py-8 text-center">
       <p className="font-mono text-sm text-ink-soft">
         © {new Date().getFullYear()} blog. All rights reserved.
       </p>