test(app): add headless ViewModel, CLI and localization tests

ViewModels run on Avalonia.Headless: wizard flows, password and file-lock
handling, seed and private-key reveal gating, contacts, settings, sync and
send against the fake server. The CLI entry point is invoked in-process
end to end. Localization checks cover the 7 languages, placeholders and
keys used in sources. Internal seams (UpdateCheck, ProfileOverride) keep
the tests offline.

tests/PalladiumWallet.Tests/App/HeadlessSetup.cs

@@ -0,0 +1,29 @@
+using Avalonia;
+using Avalonia.Headless;
+using PalladiumWallet.Tests.App;
+
+// Boots the real Avalonia Application headlessly (no window, no GPU) so ViewModels that use
+// the dispatcher (DispatcherTimer, UIThread.Post) can be tested. The xunit-v3-only helper
+// package (Avalonia.Headless.XUnit) is deliberately not used: this suite is on xunit 2.
+[assembly: AvaloniaTestApplication(typeof(TestAppBuilder))]
+
+namespace PalladiumWallet.Tests.App;
+
+public class TestAppBuilder
+{
+    public static AppBuilder BuildAvaloniaApp() =>
+        AppBuilder.Configure<global::PalladiumWallet.App.App>()
+            .UseHeadless(new AvaloniaHeadlessPlatformOptions());
+}
+
+/// <summary>One shared headless Avalonia session; tests run their body on its UI thread.</summary>
+internal static class UiSession
+{
+    private static readonly Lazy<HeadlessUnitTestSession> Session =
+        new(() => HeadlessUnitTestSession.StartNew(typeof(TestAppBuilder)));
+
+    public static Task Run(Func<Task> body) => Session.Value.Dispatch(body, CancellationToken.None);
+
+    public static Task Run(Action body) =>
+        Session.Value.Dispatch(() => { body(); return Task.CompletedTask; }, CancellationToken.None);
+}