feat(postit): wire Circle + BlogAcl clients in the DI container

App.axaml.cs is the composition root for PostIt. It now also
builds and registers:
- CircleApiClient (singleton) — backed by the same YavscApiClient
  and the same blogs base URL as BlogApiClient
- BlogAclApiClient (singleton) — same shape
- IYavscApiClient -> YavscApiClient mapping (singleton). The
  concrete class is still resolvable as YavscApiClient; the new
  registration makes the same instance available as
  IYavscApiClient so future consumers (and unit tests) can take
  the interface without coupling to the concrete type.

The 3 high-level clients are singletons: they hold no mutable
state of their own, just a reference to YavscApiClient and a
base URL. Reusing the same instance across requests is what the
HttpClient inside YavscApiClient was already designed for.

src/PostIt/PostIt/App.axaml.cs

@@ -57,6 +57,8 @@ public partial class App : Application
 
         var api = new YavscApiClient(settings, tokenStore);
         var client = new BlogApiClient(api, settings.BlogsApiUrl);
+        var circleClient = new CircleApiClient(api, settings.BlogsApiUrl);
+        var blogAclClient = new BlogAclApiClient(api, settings.BlogsApiUrl);
 
         var services = new ServiceCollection();
 
@@ -79,8 +81,11 @@ public partial class App : Application
 
         // ViewModels
         services.AddSingleton(settings);
-        services.AddSingleton(api);
+        services.AddSingleton<YavscApiClient>(api);
+        services.AddSingleton<IYavscApiClient>(api);
         services.AddSingleton(client);
+        services.AddSingleton(circleClient);
+        services.AddSingleton(blogAclClient);
         services.AddTransient<MainPageViewModel>();
         services.AddTransient<HomePageViewModel>();
         services.AddTransient<SignaturePageViewModel>();