Don't use shared background context for fetching item counts

A significant fraction of the time was spent waiting for the background
context to be available, before the count could even be started. Since
the counts don't need to use the shared background context, let them
each use their own context to avoid contention.

Reader/Screens/AppSplitViewController.swift

@@ -45,11 +45,15 @@ class AppSplitViewController: UISplitViewController {
         setViewController(sidebarNav, for: .primary)
         
         secondaryNav = UINavigationController()
+        #if targetEnvironment(macCatalyst)
+        secondaryNav.isNavigationBarHidden = true
+        #else
         // the toggle sidebar button only appears if there's a navigation bar
         // so we just make always transparent, rather than disabling it
         let secondaryNavBarAppearance = UINavigationBarAppearance()
         secondaryNavBarAppearance.configureWithTransparentBackground()
         secondaryNav.navigationBar.standardAppearance = secondaryNavBarAppearance
+        #endif
         secondaryNav.view.backgroundColor = .appBackground
         setViewController(secondaryNav, for: .secondary)