fix(postit): repair Circles bindings + align UserSearchApi route

- CirclesPage: drop VisualRoot/MainWindow hack, switch to App.PushPageAsync(vm)
- CirclesPageViewModel: make OpenAddMemberAsync public so Avalonia XAML trampoline can call it
- AddCircleMemberDialog: bind SearchAsync/Add (drop Command suffix)
- UserSearchApiController: route under Constants.APIPrefix (= api/v1/user-search), matching the rest of Yavsc.Blogs controllers and the PostIt client's BlogsApiUrl default

src/Yavsc.Blogs/Controllers/UserSearchApiController.cs

@@ -26,7 +26,7 @@ namespace Yavsc.Blogs.Controllers
     /// exposing it.</para>
     /// </summary>
     [Produces("application/json")]
-    [Route("api/user-search")]
+    [Route( Constants.APIPrefix + "/user-search")]
     [Authorize]
     public class UserSearchApiController : Controller
     {
@@ -108,4 +108,4 @@ namespace Yavsc.Blogs.Controllers
         public string? Avatar { get; set; }
         public string? Email { get; set; }
     }
-}
+}