Use a scheme-adaptive background for tvOS full screen covers

tvOS full screen covers are transparent, so each one filled itself with
a hardcoded black — invisible black-on-black text under the light color
scheme (e.g. the View All Comments page). A shared
tvFullScreenCoverBackground() modifier now picks black or light gray
from the color scheme; applied to the comments cover, subscription
destination picker, instance login, edit source, iCloud progress, and
legacy accounts import covers.

Yattee/Views/Settings/InstanceLoginView.swift

@@ -54,7 +54,7 @@ struct InstanceLoginView: View {
             formContent
         }
         .frame(maxWidth: .infinity, maxHeight: .infinity)
-        .background(Color.black.ignoresSafeArea())
+        .tvFullScreenCoverBackground()
         .accessibilityIdentifier("instance.login.view")
         #else
         NavigationStack {