refactor(ui): extract NowPlaying visibility check into a named variable

Pulls the compound "enabled && (not admin-only or is admin)" condition out
of the JSX into a canViewNowPlaying variable, so the AppBar render reads as
intent rather than a multi-operator boolean. No behavior change.

Signed-off-by: Deluan <deluan@navidrome.org>

ui/src/layout/AppBar.jsx

@@ -118,13 +118,13 @@ const CustomUserMenu = ({ onClick, ...rest }) => {
     )
   }
 
+  const canViewNowPlaying =
+    config.enableNowPlaying &&
+    (!config.nowPlayingAdminOnly || permissions === 'admin')
+
   return (
     <>
-      {config.devActivityPanel &&
-        config.enableNowPlaying &&
-        (!config.nowPlayingAdminOnly || permissions === 'admin') && (
-          <NowPlayingPanel />
-        )}
+      {config.devActivityPanel && canViewNowPlaying && <NowPlayingPanel />}
       {config.devActivityPanel && permissions === 'admin' && <ActivityPanel />}
       <UserMenu {...rest}>
         <PersonalMenu sidebarIsOpen={true} onClick={onClick} />