fix: make panel top nav menu item button full width on mobile (#200823)

## Summary

This PR solves [[Dashboard] Center Save top nav button for small
viewport](#180093 (comment)) issue.

![Screenshot 2024-11-19 at 23 02
16](https://github.com/user-attachments/assets/ac643c61-62d5-4512-af35-33c396f6aba1)

![Screenshot 2024-11-19 at 23 01
31](https://github.com/user-attachments/assets/4d2021fb-a08b-4626-a203-07b52f3cbe55)

The change only affects the mobile menu.

src/plugins/navigation/public/top_nav_menu/top_nav_menu_item.tsx

@@ -80,7 +80,12 @@ export function TopNavMenuItem(props: TopNavMenuData & { isMobileMenu?: boolean
       </EuiToolTip>
     ) : props.emphasize ? (
       // fill is not compatible with EuiHeaderLink
-      <EuiButton size="s" {...commonButtonProps} fill={props.fill ?? true}>
+      <EuiButton
+        size="s"
+        fullWidth={props.isMobileMenu}
+        {...commonButtonProps}
+        fill={props.fill ?? true}
+      >
         {getButtonContainer()}
       </EuiButton>
     ) : (