[client] Use WinRT COM for Windows toasts (#6013)

* Use WinRT COM for Windows toasts instead of fyne's PowerShell path

* Quote autostart path and split HKCU registry into per-user component

client/ui/notifier/notifier.go

@@ -0,0 +1,27 @@
+// Package notifier sends desktop notifications. On Windows it uses the WinRT
+// COM API directly via go-toast/v2 to avoid the PowerShell window flash that
+// fyne's default implementation produces. On other platforms it delegates to
+// fyne.
+package notifier
+
+import "fyne.io/fyne/v2"
+
+// Notifier sends desktop notifications.
+type Notifier interface {
+	Send(title, body string)
+}
+
+// New returns a platform-specific Notifier. The fyne app is used as the
+// fallback notifier on platforms where no native implementation is wired up,
+// and on Windows when the COM path fails to initialize.
+func New(app fyne.App) Notifier {
+	return newNotifier(app)
+}
+
+type fyneNotifier struct {
+	app fyne.App
+}
+
+func (f *fyneNotifier) Send(title, body string) {
+	f.app.SendNotification(fyne.NewNotification(title, body))
+}