Weird rounded corners under KDE6/Wayland with AUI_PA=gtk4

### One More Step Before opening this issue

- [x] To follow our [Code of Conduct](https://github.com/aui-framework/aui/blob/master/CODE_OF_CONDUCT.md)
- [x] I've considered [existing issues on GitHub page](https://github.com/aui-framework/aui/issues) and didn't find a similar issue

### What happened?

I'm not sure gtk4 is supposed to be used without libadwaita though. Folks from gtk/gnome don't like server side decorations, despite support of server side decorations wayland protocol is implemented in gtk.

Possible fix is to adjust `border-radius` of the window.

### Relevant log output or stack trace

```shell

```

### Steps to reproduce

```bash
(under KDE6+Wayland)

AUI_PA=gtk4 ./aui.example.views
```

### Affected Platforms

Linux

### Link to build, i.e. failing GitHub Action job (if any).

_No response_

Reproduce the issue under KDE6+Wayland by running `AUI_PA=gtk4 ./aui.example.views`. Start by tracing the GTK4 window-decoration and `border-radius` handling involved in that entry point, then verify that the rounded corners render correctly without introducing regressions on the affected Linux setup.

Written by the indexing model from the issue text.