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/Video/VideoInfoView.swift

@@ -1989,9 +1989,7 @@ struct VideoInfoView: View {
                         }
                     }
                 }
-                #if os(tvOS)
-                .background(Color.black.ignoresSafeArea())
-                #endif
+                .tvFullScreenCoverBackground()
                 .navigationTitle(String(localized: "videoInfo.section.comments"))
                 #if os(iOS)
                 .navigationBarTitleDisplayMode(.inline)