feat: display article summary in header

- Add summary field to ReadableContent interface
- Pass summary through ContentPanel to ReaderHeader
- Display summary below title in both overlay and standard layouts
- Style summary with reading font for consistency
- Summary appears in white with shadow in image overlays
- Summary appears in gray (#aaa) in standard headers
- Enhances article preview and reading experience

src/components/ThreePaneLayout.tsx

@@ -105,6 +105,7 @@ const ThreePaneLayout: React.FC<ThreePaneLayoutProps> = (props) => {
               html={props.readerContent?.html}
               markdown={props.readerContent?.markdown}
               image={props.readerContent?.image}
+              summary={props.readerContent?.summary}
               selectedUrl={props.selectedUrl}
               highlights={props.classifiedHighlights}
               showHighlights={props.showHighlights}