feat(app): check GitHub releases on startup and prompt for updates

Compares the running assembly version against the latest GitHub release
tag on every app launch (desktop and Android) and shows an in-app
overlay with the new version tag when one is available. Best-effort:
network/parse failures are silent, matching the app's existing
non-blocking startup checks.

src/App/Views/MainView.axaml.cs

@@ -129,6 +129,13 @@ public partial class MainView : UserControl
             vm.IsHelpOpen = false;
     }
 
+    private void OnUpdateAvailableOverlayBackdropTapped(object? sender, TappedEventArgs e)
+    {
+        if (!ReferenceEquals(e.Source, sender)) return;
+        if (DataContext is MainWindowViewModel vm)
+            vm.IsUpdateAvailableOpen = false;
+    }
+
     private async void OnOpenBugReportClick(object? sender, RoutedEventArgs e)
     {
         const string issueUrl = "https://github.com/davide3011/PalladiumWallet/issues/new?template=bug_report.yml";
@@ -137,6 +144,15 @@ public partial class MainView : UserControl
             await launcher.LaunchUriAsync(new Uri(issueUrl));
     }
 
+    private async void OnOpenReleasePageClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not MainWindowViewModel vm || string.IsNullOrEmpty(vm.UpdateReleaseUrl)) return;
+        var launcher = TopLevel.GetTopLevel(this)?.Launcher;
+        if (launcher is not null)
+            await launcher.LaunchUriAsync(new Uri(vm.UpdateReleaseUrl));
+        vm.IsUpdateAvailableOpen = false;
+    }
+
     // Esc (desktop) or Back (Android) closes the topmost open overlay.
     protected override void OnKeyDown(KeyEventArgs e)
     {
@@ -149,6 +165,7 @@ public partial class MainView : UserControl
             if (vm.IsWalletInfoOpen) { vm.CloseWalletInfoCommand.Execute(null); e.Handled = true; return; }
             if (vm.IsSettingsOpen) { vm.IsSettingsOpen = false; e.Handled = true; return; }
             if (vm.IsHelpOpen) { vm.IsHelpOpen = false; e.Handled = true; return; }
+            if (vm.IsUpdateAvailableOpen) { vm.IsUpdateAvailableOpen = false; e.Handled = true; return; }
         }
         base.OnKeyDown(e);
     }