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/ViewModels/MainWindowViewModel.cs

@@ -158,6 +158,7 @@ public partial class MainWindowViewModel : ViewModelBase
         _keepAliveTimer = new DispatcherTimer { Interval = System.TimeSpan.FromSeconds(20) };
         _keepAliveTimer.Tick += async (_, _) => await KeepAliveTickAsync();
         _keepAliveTimer.Start();
+        _ = CheckForUpdatesAsync();
     }
 
     private async System.Threading.Tasks.Task KeepAliveTickAsync()