fix: show highlight button for all reading content

- Show highlight button when readerContent exists (both nostr articles and external URLs)
- Hide highlight button when browsing app pages like explore, settings, etc.
- Ensures highlighting is available for all readable content but not for navigation pages

src/components/ThreePaneLayout.tsx

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