valhalla/web-app ? reverse-engineered prompt

Build me a map based web app like the Valhalla demo site, where I can plan routes and draw travel time areas on an OpenStreetMap map.

I want it to let me pick a start point, choose how I am traveling, and show the route results clearly on the map with useful options I can tweak. It should also support isochrones, so I can see how far I can get within a certain time or distance. Make it feel polished and easy to use, with a good default map view and simple controls.

Please set it up so the backend API endpoints and map settings can be changed from environment values, including the Valhalla server, the search service, the tile server, and the default starting location. Keep it runnable locally for development and buildable for production. If you need current docs or examples for the APIs, look them up online and use the latest info.