Test: Once Option

What this tests:

The once: true option - handler executes exactly once then auto-unregisters.

Expected behavior:

• 1st back → Dialog appears, count shows 1
• Click "Cancel" → Dialog closes, handler is already unregistered
• 2nd back → preRegisteredHandler closes any open dialog
• 3rd back → No handler, navigates to Home

Key point:

"once" means "execute once" regardless of return value, not "allow navigation once"