postIt: wire DarkMode, drop dead themeVariant, add UI tests for the banner

Three related changes that close the loop on the DarkMode
field and lay the first stone of a UI test scaffold for
PostIt.

1. Settings.DarkMode was previously a dead field. It round-
   tripped through postit-settings.json and the SettingsPage
   CheckBox, OnDarkModeChanged flipped IsDirty, and that was
   it — no consumer ever read the value, so toggling the
   CheckBox had no visible effect. The fix is in
   App.OnFrameworkInitializationCompleted: read the value
   Load() just populated and set
   Application.Current.RequestedThemeVariant accordingly
   (so a dark-mode user lands on a dark window on first
   launch, not on a default-light window that flips after
   the user touches the toggle), then subscribe to
   settings.PropertyChanged and update the theme on every
   DarkMode change. The consumer lives in App.axaml.cs, not
   in Settings, so the Settings model stays free of any
   Avalonia.Application dependency and the SettingsLoadTests
   (which construct Settings outside an Avalonia host)
   still pass unchanged.

2. MainPageViewModel had a vestigial [ObservableProperty]
   ThemeVariant themeVariant = ThemeVariant.Default that no
   XAML, no code, and no test ever read. It was the start of
   a half-finished attempt to expose the theme variant on
   the page VM. The dark-mode wiring above makes it
   irrelevant: the theme is now driven by Application, not
   by a VM property. The field is removed, along with the
   using Avalonia.Styling; it pulled in (now unused).

3. SessionStatusBannerTests adds the first set of UI tests
   for PostIt. They mount a real MainWindow via the headless
   Avalonia host declared in TestApp.cs, attach a
   SessionStatusViewModel as the banner's DataContext, and
   assert the actual visual tree contents: three buttons
   render (Se déconnecter, Se connecter, Paramètres), the
   Login button is visible when logged out, the Logout
   button is hidden when logged out, the Paramètres button
   is visible regardless of session, and the session label
   text reflects the VM. The pattern follows what
   UnitTest1.MainPage_Should_Load already established:
   [AvaloniaFact] (from Avalonia.Headless.XUnit) plus
   new MainWindow() / window.Show(). A plain [Fact] cannot
   drive Window..ctor() because the headless platform's
   PlatformManager.CreateWindow() has no service registered
   outside a dispatcher-aware test context; the
   AvaloniaFact attribute provides that context. The
   DataContext is set on the banner directly because
   App.OnFrameworkInitializationCompleted is not called in
   a unit test (production wiring is exercised by the
   manual launch, not here).

Build: 0 errors. Tests: 5/5 SessionStatusBannerTests,
3/3 SettingsLoadTests, 1/1 MainPageTests (the existing
scaffold test, unchanged). The other PostIt.Tests suites
depend on the OIDC stub WebApplicationFactory and time out
on this network-restricted host.

src/PostIt/PostIt/App.axaml.cs

@@ -5,6 +5,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
+using Avalonia.Styling;
 using PostIt.Services;
 using PostIt.ViewModels;
 using PostIt.Views;
@@ -112,8 +113,24 @@ public partial class App : Application
         // DataContext, and the TwoWay bindings inside the page keep
         // mutating the same in-memory Settings instance that the rest
         // of the app reads (OidcClientOptions construction, etc.).
-        provider.GetRequiredService<SettingsPage>().DataContext =
-            provider.GetRequiredService<Settings>();
+        provider.GetRequiredService<SettingsPage>().DataContext = settings;
+
+        // Settings.DarkMode was previously a dead field: it round-
+        // tripped through the settings file and the SettingsPage
+        // CheckBox, but no consumer ever read it. Wire it here to
+        // Application.RequestedThemeVariant so the toggle takes
+        // effect immediately, and seed the initial theme from the
+        // value Load() just populated (so a dark-mode user lands on
+        // a dark window on first launch, not on a default-light
+        // window that flips after the user touches the toggle).
+        ApplyDarkMode(settings);
+        settings.PropertyChanged += (_, e) =>
+        {
+            if (e.PropertyName == nameof(Settings.DarkMode))
+            {
+                ApplyDarkMode(settings);
+            }
+        };
 
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
@@ -190,6 +207,12 @@ public partial class App : Application
         }
     }
 
+    private static void ApplyDarkMode(Settings settings)
+    {
+        Application.Current!.RequestedThemeVariant =
+                            settings.DarkMode ? ThemeVariant.Dark : ThemeVariant.Light;
+    }
+
     /// <summary>
     /// Run once after the main window is shown: try to refresh the
     /// cached OIDC tokens silently; on success, push MainPage on top

src/PostIt/PostIt/ViewModels/MainPageViewModel.cs

@@ -2,7 +2,6 @@ using System;
 using System.Collections.ObjectModel;
 using System.Linq;
 using System.Threading.Tasks;
-using Avalonia.Styling;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using PostIt.Models;
@@ -38,9 +37,6 @@ public partial class MainPageViewModel : ViewModelBase
     [ObservableProperty]
     public partial bool IsBusy { get; set; }
 
-    [ObservableProperty]
-    ThemeVariant themeVariant = ThemeVariant.Default;
-
     [ObservableProperty]
     public partial Settings Settings { get; private set; }