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.

src/App/ViewModels/MainWindowViewModel.Update.cs

@@ -20,13 +20,17 @@ public partial class MainWindowViewModel
     [RelayCommand]
     private void CloseUpdateAvailable() => IsUpdateAvailableOpen = false;
 
+    /// <summary>Test seam: replaces the real GitHub check so tests never touch the network.</summary>
+    internal static System.Func<string, Task<LatestRelease?>> UpdateCheck { get; set; } =
+        version => UpdateChecker.CheckAsync(version);
+
     /// <summary>
     /// Fire-and-forget check run once at startup. Best-effort: any failure or an
     /// up-to-date app silently does nothing (see <see cref="UpdateChecker"/>).
     /// </summary>
     private async Task CheckForUpdatesAsync()
     {
-        var latest = await UpdateChecker.CheckAsync(AppVersion);
+        var latest = await UpdateCheck(AppVersion);
         if (latest is null) return;
 
         UpdateReleaseUrl = latest.HtmlUrl;

src/App/ViewModels/MainWindowViewModel.cs

@@ -130,7 +130,10 @@ public partial class MainWindowViewModel : ViewModelBase
     // ---- helpers ----
 
     private NetKind Net => NetKind.Mainnet;
-    private ChainProfile Profile => ChainProfiles.For(Net);
+    /// <summary>Test seam: lets tests drop the real bootstrap servers so nothing reaches the internet.</summary>
+    internal static ChainProfile? ProfileOverride { get; set; }
+
+    private ChainProfile Profile => ProfileOverride ?? ChainProfiles.For(Net);
     private ServerRegistry Registry => new(Profile, AppPaths.ServersPath(Net));
 
     private string Fmt(long sats, bool withLabel = true) =>