Fix tvOS onboarding background transparency and button styling

Add opaque black background to onboarding on tvOS to prevent Home
screen content from leaking through the fullScreenCover. Replace
toolbar Skip button with plain overlay button to avoid blurred
material style, and add tvOS card button style with default focus
on Continue button.

Yattee/Views/Onboarding/OnboardingSheetView.swift

@@ -63,7 +63,19 @@ struct OnboardingSheetView: View {
         #elseif os(iOS)
         .tabViewStyle(.page(indexDisplayMode: .never))
         #endif
+        #if os(tvOS)
+        .background(Color.black.ignoresSafeArea())
+        .overlay(alignment: .topLeading) {
+            Button(String(localized: "onboarding.skip")) {
+                completeOnboarding()
+            }
+            .buttonStyle(.plain)
+            .foregroundStyle(.secondary)
+            .padding(40)
+        }
+        #endif
         .interactiveDismissDisabled()
+        #if !os(tvOS)
         .toolbar {
             ToolbarItem(placement: .cancellationAction) {
                 Button(String(localized: "onboarding.skip")) {
@@ -71,6 +83,7 @@ struct OnboardingSheetView: View {
                 }
             }
         }
+        #endif
         .task {
             guard !hasCheckedLegacyData else { return }
             hasCheckedLegacyData = true