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/Components/SubscriptionDestinationMenu.swift

@@ -367,8 +367,6 @@ struct SubscriptionDestinationPicker: View {
 
     var body: some View {
         ZStack {
-            Color.black.ignoresSafeArea()
-
             VStack(alignment: .leading, spacing: 24) {
                 Text(String(localized: "channel.menu.subscribeOn"))
                     .font(.title2.bold())
@@ -395,6 +393,8 @@ struct SubscriptionDestinationPicker: View {
             .padding(.horizontal, 80)
             .padding(.vertical, 60)
         }
+        .frame(maxWidth: .infinity, maxHeight: .infinity)
+        .tvFullScreenCoverBackground()
     }
 }
 #endif