fix: reset desktop.ShutdownMode to ShutdownMode.OnExplicitShutdown before calling desktop.Shutdown manually (#2129)

Signed-off-by: leo <longshuang@msn.cn>

src/App.axaml.cs

@@ -387,6 +387,7 @@ namespace SourceGit
         {
             if (Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
             {
+                desktop.ShutdownMode = ShutdownMode.OnExplicitShutdown;
                 desktop.MainWindow?.Close();
                 desktop.Shutdown(exitCode);
             }