PostIt: route Paramètres button to SettingsPage

The "Paramètres" button on SessionStatusBanner was wired to a stub
OpenSettingsCommand with a TODO. With the Settings model refactor
(VM consolidated to ViewModels/Settings.cs, SettingsViewModel.cs
dropped, App.axaml.cs registering Settings instead of the old VM),
the navigation is now plumbed end to end:

- SessionStatusViewModel gains an OpenSettingsRequested event
  alongside LogoutCompleted / LoginSucceeded, and the
  [RelayCommand] body just raises it. VM stays decoupled from
  NavigationPage and window lifetime, same pattern as the
  existing banner events.
- App.axaml.cs handles the event in the desktop branch: resolves
  SettingsPage (transient) and the canonical Settings singleton
  (the one we Load()'d at startup and bound via
  Settings.BindToServiceProvider) from DI, then PushAsync the
  page on top of the current NavRoot stack. Two-way bindings on
  SettingsPage mutate the singleton in place.

Build: dotnet build src/PostIt/PostIt/PostIt.csproj → 0 errors.
Existing CS8602 / NU1507 / CS8632 warnings unchanged.

src/PostIt/PostIt/Services/BlogApiClient.cs

@@ -40,6 +40,11 @@ public sealed class BlogApiClient
     public BlogApiClient(YavscApiClient api, string pathPrefix = DefaultPathPrefix)
     {
         _api = api ?? throw new ArgumentNullException(nameof(api));
+
+        // ApiUrl is e.g. "https://blogs.pschneider.fr/api/v1/" — keep the
+        // trailing slash so relative paths ("posts") resolve correctly.
+        api.Http.BaseAddress = new Uri(api.Settings.BusinessApiUrl);
+
         _pathPrefix = pathPrefix?.TrimStart('/') ?? DefaultPathPrefix;
     }