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

Yattee/Views/Components/TVFullScreenCoverBackground.swift

@@ -0,0 +1,39 @@
+//
+//  TVFullScreenCoverBackground.swift
+//  Yattee
+//
+//  Scheme-adaptive opaque background for tvOS full screen covers.
+//
+
+import SwiftUI
+
+#if os(tvOS)
+/// tvOS full screen covers have a transparent background, letting the view
+/// below leak through, so they need an explicit opaque fill. A hardcoded
+/// black makes primary (black) text invisible under the light color scheme,
+/// so the fill follows the scheme instead.
+struct TVFullScreenCoverBackgroundModifier: ViewModifier {
+    @Environment(\.colorScheme) private var colorScheme
+
+    func body(content: Content) -> some View {
+        content
+            .background(
+                (colorScheme == .dark ? Color.black : Color(white: 0.92))
+                    .ignoresSafeArea()
+            )
+    }
+}
+#endif
+
+extension View {
+    /// Gives a tvOS full screen cover an opaque, color-scheme-appropriate
+    /// background. No-op on other platforms.
+    @ViewBuilder
+    func tvFullScreenCoverBackground() -> some View {
+        #if os(tvOS)
+        modifier(TVFullScreenCoverBackgroundModifier())
+        #else
+        self
+        #endif
+    }
+}

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) {

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 {

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)