Offer a delayed Cancel while tvOS external video extraction loads

A screen showing only a spinner has nothing focusable, so a slow or hung
extraction stranded the tvOS remote. Add TVStalledLoadCancelButton, a
Cancel that appears under a loading spinner after three seconds so
normal loads never flash it, and use it for external video extraction.

Yattee/Views/Components/LoadStateView.swift

@@ -13,6 +13,8 @@ import SwiftUI
 struct LoadingView: View {
     var message: String? = nil
     var subtext: String? = nil
+    /// tvOS: offered as a Cancel button once loading has taken a while.
+    var onCancel: (() -> Void)? = nil
 
     var body: some View {
         VStack(spacing: 16) {
@@ -32,11 +34,59 @@ struct LoadingView: View {
                     .truncationMode(.middle)
                     .padding(.horizontal)
             }
+
+            #if os(tvOS)
+            if let onCancel {
+                TVStalledLoadCancelButton(action: onCancel)
+                    .padding(.top, 24)
+            }
+            #endif
         }
         .frame(maxWidth: .infinity, maxHeight: .infinity)
     }
 }
 
+#if os(tvOS)
+// MARK: - Stalled Load Cancel
+
+/// A Cancel button that appears under a tvOS loading spinner once the load has
+/// taken a while.
+///
+/// A screen showing only a spinner has nothing focusable: the focus engine has
+/// nowhere to land, the remote cannot reach the sidebar and Menu cannot pop the
+/// screen, so a hung request strands the user until it times out. Delayed so
+/// a normal load never flashes the button.
+struct TVStalledLoadCancelButton: View {
+    var delay: Duration = .seconds(3)
+    let action: () -> Void
+
+    @State private var isShown = false
+
+    var body: some View {
+        Group {
+            if isShown {
+                Button(String(localized: "common.cancel"), action: action)
+            }
+        }
+        .task {
+            guard (try? await Task.sleep(for: delay)) != nil else { return }
+            isShown = true
+        }
+    }
+}
+
+extension View {
+    /// Puts a delayed Cancel under a tvOS loading spinner. See
+    /// `TVStalledLoadCancelButton`.
+    func tvStalledLoadCancel(action: @escaping () -> Void) -> some View {
+        VStack(spacing: 40) {
+            self
+            TVStalledLoadCancelButton(action: action)
+        }
+    }
+}
+#endif
+
 // MARK: - Error State View
 
 /// An error view with icon, message, and action buttons.

Yattee/Views/Video/ExternalVideoView.swift

@@ -24,7 +24,8 @@ struct ExternalVideoView: View {
             if isLoading {
                 LoadingView(
                     message: String(localized: "externalVideo.extracting"),
-                    subtext: url.host ?? url.absoluteString
+                    subtext: url.host ?? url.absoluteString,
+                    onCancel: { dismiss() }
                 )
             } else if let error = errorMessage {
                 ErrorStateView(