postit: drop ConfigureAwait(false) after interactive login

LoginAsync uses ConfigureAwait(false) on the await of
LoginInteractiveCoreAsync. Since the surrounding code is
already executing on the UI thread (it was reached via a
RelayCommand that the UI dispatcher dispatched), the
ConfigureAwait drops the SynchronizationContext, and the
subsequent setters — IsBusy, AccessToken, StatusMessage,
LoginSuccess, and the LoginSucceeded?.Invoke() — run on a
thread-pool worker.

The downstream effects are all UI-bound: PropertyChanged
events fire, the BindingEngine republishes them as
AvaloniaObject.SetValue calls, and SetValue calls
Dispatcher.VerifyAccess. VerifyAccess throws because the
AvaloniaObject was created on the UI thread (owned by it)
and the SetValue is being attempted from a thread-pool
worker. Avalonia 11.12 throws SynchronousException through
DispatcherOperation.InvokeCore instead of dispatching back,
so the X11 message loop crashes the process with
System.InvalidOperationException: 'The calling thread cannot
access this object because a different thread owns it.'

Reproduced with the freshly installed postit_1.0.0-1_amd64.deb
package on a Debian 13 host — the .NET runtime loaded the
app, Avalonia started the X11 message loop, the operator
clicked 'Se connecter', the OIDC flow reached the post-login
phase, and the post-await setter chain crashed the process.

Drop ConfigureAwait(false) so the await captures the UI
thread SynchronizationContext and the setters resume on the
UI thread. The inner LoginInteractiveCoreAsync still uses
ConfigureAwait(false) for its own await, which is fine —
the inner method does not touch observables, only mutates
Platform.CreateBrowser and awaits the OIDC roundtrip, so it
can run anywhere.

src/PostIt/PostIt/ViewModels/LoginPageViewModel.cs

@@ -262,7 +262,7 @@ public partial class LoginPageViewModel : ViewModelBase
             // keeps the text detail (URLs, error messages). Same
             // underlying flow, two views.
             var progress = new Progress<OidcLoginPhase>(p => Phase = p);
-            await LoginInteractiveCoreAsync(_api, progress).ConfigureAwait(false);
+            await LoginInteractiveCoreAsync(_api, progress);
 
             IsBusy = false;
             AccessToken = _api.CurrentAccessToken;