style: improve publishing date display

- Use shorter date format (MMM d, yyyy instead of MMMM d, yyyy)
- Add subtle styling with reduced opacity and smaller font
- Make calendar icon smaller and more muted
- Style overlay version for hero images with subtle white text

src/components/ReaderHeader.tsx

@@ -22,7 +22,7 @@ const ReaderHeader: React.FC<ReaderHeaderProps> = ({
   hasHighlights,
   highlightCount
 }) => {
-  const formattedDate = published ? format(new Date(published * 1000), 'MMMM d, yyyy') : null
+  const formattedDate = published ? format(new Date(published * 1000), 'MMM d, yyyy') : null
   if (image) {
     return (
       <div className="reader-hero-image">