fix: replace 'No readable content found' with skeleton loader

Replace the confusing 'No readable content found for this URL' message that
appears during loading states with a skeleton loader for better UX.

This prevents users from seeing error messages while content is still loading.

src/components/ContentPanel.tsx

@@ -976,8 +976,8 @@ const ContentPanel: React.FC<ContentPanelProps> = ({
           )}
         </>
       ) : (
-        <div className="reader empty">
+        <div className="reader" aria-busy="true">
-          <p>No readable content found for this URL.</p>
+          <ContentSkeleton />
         </div>
       )}
       </div>