refactor(ui): move logout button to top-right of app

- Move logout IconButton from sidebar to App component
- Position logout button fixed at top-right corner
- Remove onLogout prop from Bookmarks and BookmarkList components
- Clean up sidebar header by removing logout button
- Add app-header CSS with fixed positioning and high z-index

src/index.css

@@ -28,6 +28,14 @@ body {
 
 .app {
   text-align: center;
+  position: relative;
+}
+
+.app-header {
+  position: fixed;
+  top: 1rem;
+  right: 2rem;
+  z-index: 1000;
 }
 
 .app header {