Add to Favourites Endpoint

[refokcer]

Task: Create a MediatR handler that allows users to add a Streetcode to their favourite list. Steps:

1. Create a AddStreetcodeToFavouritesCommand.
2. Implement a AddStreetcodeToFavouritesHandler.
3. Store the favourite list in the browser's localStorage.
4. Create a controller action to handle adding to favourites.