[GUI] Closing the GUI multiple times while being prompted stacks indefinitely

**Describe the bug**
When pressing the exit button from the GUI with a running instance and the option set to ask to whether stop or keep the instance running; the intended popup shows up and the GUI darkens slightly. If you press the button repeatedly, more popups will stack (which can be discovered when closing the existing one) and the GUI will continuously darken until it is completely black (the popups will still be visible).

**To Reproduce**
In any snap or build
1. `multipass.gui`
2. Get one instance running.
3. Change settings to prompt the user to stop instances.
4. Press the window close "x" button multiple times.

**Expected behavior**
Either prevent multiple popups, emit a tone and refocus the popup, or similar. No multiple stacking and progressively darkening of the GUI.

**Additional info**
- OS: Ubuntu 24.04
- CPU architecture or model: x86
- Any available version, both locally built or snaps.

Reproduce the issue with `multipass.gui` using a running instance and the setting that prompts before stopping instances. Trace the GUI window-close handling and prompt creation; done means repeated close-button presses leave only one prompt visible and do not progressively darken the GUI.

Written by the indexing model from the issue text.