feat: show highlight button only when viewing articles

- Only display the floating highlight button when currentArticle exists or selectedUrl is a nostr article
- Prevents highlight button from showing on external URLs, videos, or other content types
- Improves UX by showing highlight functionality only where it's relevant

src/components/ThreePaneLayout.tsx

@@ -414,7 +414,7 @@ const ThreePaneLayout: React.FC<ThreePaneLayoutProps> = (props) => {
           />
         </div>
       </div>
-      {props.hasActiveAccount && (
+      {props.hasActiveAccount && (props.currentArticle || (props.selectedUrl && props.selectedUrl.startsWith('nostr:'))) && (
         <HighlightButton 
           ref={props.highlightButtonRef} 
           onHighlight={props.onCreateHighlight}