[BUG] Popup - Background color - Not fully transparent

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Did you read the "Reporting a bug" section on Contributing file?

- [x] I have read the "Reporting a bug" section on Contributing file: https://github.com/CommunityToolkit/Maui/blob/main/CONTRIBUTING.md#reporting-a-bug

### Current Behavior

Setting popup overlay color to transparent doesn't make it fully transparent, it is still dimmed.

### Expected Behavior

Expecting background overlay color to be transparent, but it is a bit dimmed.

### Steps To Reproduce

Set background to transparent color:

```

```

```
private async void Button_Menu_Clicked(object sender, EventArgs e)
{
var options = new PopupOptions { Shape = null, Shadow = null, PageOverlayColor = Colors.Transparent };
await Application.Current!.Windows[0].Page!.ShowPopupAsync(new SideMenu(), options);
}
```

### Link to public reproduction project repository

.

### Environment

```markdown
- .NET MAUI CommunityToolkit: 14
- OS: Windows / Adnroid
- .NET MAUI: 10
```

### Anything else?

_No response_

Start with the PopupOptions.PageOverlayColor path used by ShowPopupAsync and the Popup BackgroundColor behavior in the provided XAML reproduction. Reproduce on Windows and Android with both values set to Transparent, then verify that the popup overlay is fully transparent without dimming the underlying page.

Written by the indexing model from the issue text.