[Windows 11] Window corner preference

[lijy91]

On Windows 11, top-level windows get rounded corners from DWM. Some apps need square corners, or small rounded ones, while keeping the native title bar.

Proposal: Window::SetCornerPreference(...) backed by DwmSetWindowAttribute(DWMWA_WINDOW_CORNER_PREFERENCE) (default / do-not-round / round / round-small). It's a no-op elsewhere; on macOS, SetShape() or a hidden title bar are the alternatives.

Background:

• windows: do not show border radius leanflutter/window_manager#470: do not show border radius on Windows