chore: set app website

Mastodon/Service/APIService/APIService+App.swift

@@ -19,11 +19,13 @@ extension APIService {
     private static let clientName = "Mastodon for iOS"
     #endif
 
+    private static let appWebsite = "https://joinmastodon.org/apps"
+
     func createApplication(domain: String) -> AnyPublisher<Mastodon.Response.Content<Mastodon.Entity.Application>, Error> {
         let query = Mastodon.API.App.CreateQuery(
             clientName: APIService.clientName,
             redirectURIs: MastodonAuthenticationController.callbackURL,
-            website: nil
+            website: APIService.appWebsite
         )
         return Mastodon.API.App.create(
             session: session,