Support full-window content layouts with decorated title bar

[HansHolz09]

⚠️ Is your feature request related to a problem? Please describe

At the moment, DecoratedWindow always lays out the main Compose content below the title bar area.

This makes it impossible to build layouts where the window content continues behind or underneath the title bar, while still keeping the decorated window behavior and native window controls where applicable.

💡 Describe the solution you'd like

I would like Nucleus Decorated Window to support a mode where the Compose content can continue drawing underneath the title bar area.

The exact implementation is not important as long as the following becomes possible:

• the main window content can extend through the full window height, including the area behind or under the title bar
• native window controls and platform-specific behavior continue to work correctly
• dragging, hit testing, control button placement, and content insets remain handled correctly

In other words, this request is about enabling full-window content layouts without losing the existing decorated-window behavior.

🤚 Do you want to develop this feature yourself?

• Yes
• No

[kdroidFilter]

I created it on macOS here https://github.com/kdroidFilter/compose-macos-26-ui , I don't know if I'll integrate it into the core of Nucleus; for now, it only works on macOS.