add a sse demo

src/frontend/components/nav.rs

@@ -42,6 +42,7 @@ pub fn nav_bar() -> Html {
     for (route, title) in [
         (Route::Home, "Home"),
         (Route::Counter, "Counter"),
+        (Route::Echo, "Echo"),
     ] {
         let active_item = if let Some(ref location) = maybe_location {
             let path = location.path();