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/Instances/InstanceBrowseView.swift

@@ -552,7 +552,7 @@ struct InstanceBrowseView: View {
             TVSidebarDetailContainer(systemImage: "pencil.circle", title: String(localized: "sources.editSource")) {
                 EditSourceView(source: .remoteServer(instance))
             }
-            .background(Color.black.ignoresSafeArea())
+            .tvFullScreenCoverBackground()
         }
         #else
         .sheet(isPresented: $showEditInstance) {