[client] Exit GUI immediately on Windows session end

Wails v3 runs the full app teardown synchronously inside WM_ENDSESSION,
overrunning the 5s end-session budget and triggering the "app is
preventing shutdown" screen with a forced kill. Intercept
WM_QUERYENDSESSION/WM_ENDSESSION and exit at once instead, and suppress
error dialogs, toasts, and the hide-on-close hooks once shutdown or a
tray quit has begun.

client/ui/services/shutdown.go

@@ -0,0 +1,17 @@
+package services
+
+import "sync/atomic"
+
+var shuttingDown atomic.Bool
+
+func BeginShutdown() {
+	shuttingDown.Store(true)
+}
+
+func AbortShutdown() {
+	shuttingDown.Store(false)
+}
+
+func ShuttingDown() bool {
+	return shuttingDown.Load()
+}

client/ui/services/windowmanager.go

@@ -154,6 +154,9 @@ func NewWindowManager(app *application.App, mainWindow *application.WebviewWindo
 	})
 	// Hide (not destroy) on close to keep React state; reset to General for a flash-free reopen.
 	s.settings.RegisterHook(events.Common.WindowClosing, func(e *application.WindowEvent) {
+		if ShuttingDown() {
+			return
+		}
 		e.Cancel()
 		s.app.Event.Emit(EventSettingsOpen, "general")
 		s.settings.Hide()
@@ -393,6 +396,9 @@ func (s *WindowManager) CloseWelcome() {
 // OpenError shows the custom error dialog; title/message are pre-localised and ride in the
 // start URL. A second error replaces the open one via SetURL. Singleton, destroyed on close.
 func (s *WindowManager) OpenError(title, message string) {
+	if ShuttingDown() {
+		return
+	}
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	startURL := errorDialogURL(title, message)