Fix ContentUnavailableView centering on Apple TV

On tvOS, ContentUnavailableView inside a Group doesn't expand to fill
available space — it sizes to content and aligns top-leading. Add
.frame(maxWidth: .infinity, maxHeight: .infinity) to all instances
so they center correctly in their parent containers.

Yattee/Views/Video/PlaylistSelectorSheet.swift

@@ -74,6 +74,7 @@ struct PlaylistSelectorSheet: View {
                             } description: {
                                 Text(String(localized: "playlist.empty.description"))
                             }
+                            .frame(maxWidth: .infinity, maxHeight: .infinity)
                         }
                     }
                 }