Provide activeRoute Binding

This would, for instance, allow setting the `WindowGroup` title based on the current Route:

```swift
struct MyApp : App {
@State private var activeRoute: AppRoutes = .defaultRoute
var body: some Scene {
WindowGroup(title(for: activeRoute)) {
Router(activeRoute: $activeRoute) {
...
}
}
}
}
```
Can also open the possibility of navigating programmatically:

```swift
struct MyApp : App {
@State private var activeRoute: AppRoutes = .defaultRoute
var body: some Scene {
WindowGroup {
Router(activeRoute: $activeRoute) {
...
}
}.onOpenUrl {
activeRoute = ...
}
}
}
```

Start by reading the existing Router API and the SwiftUI WindowGroup usage shown in the issue. Trace how the current route is stored and propagated, then verify that the requested activeRoute binding supports title updates and programmatic changes through onOpenUrl.

Written by the indexing model from the issue text.