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/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
+    }
+}