Fix exception when closing main window with active child windows

[Copilot]

This PR fixes a critical issue where closing the main application window while child windows are still open would throw a winrt::hresult_error exception with the message "The given object has already been closed / disposed and may no longer be used."

Problem

The issue occurred in two scenarios:

1. Creating child windows via the "Multiple windows" page and then closing the main window
2. Creating TabView tear-out windows and then closing the main window

When the main window closed, the application would begin its shutdown process while child windows were still trying to access shared resources that were being disposed, causing resource disposal conflicts.

Solution

This fix implements proper window lifecycle coordination:

App.xaml.cs: Enhanced the main window's Closed event handler to automatically close all tracked child windows when the main window closes, preventing resource conflicts during application shutdown.

CreateMultipleWindowsPage.xaml.cs: Added proper window tracking for child windows created through the "Create new Window" button by calling WindowHelper.TrackWindow().

The fix leverages the existing WindowHelper infrastructure and ensures that:

• All child windows are properly tracked and cleaned up
• Window closure happens in the correct order to prevent resource conflicts
• Exception handling protects against cascading failures during cleanup
• All existing functionality is preserved

Testing

Verified that the fix resolves the exception in both scenarios while maintaining all existing application behavior. Child windows are now automatically closed when the main window closes, providing a clean user experience without exceptions.

Fixes #1872.

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[niels9001]

@AndrewKeepCoding I'd love to get your feedback on this PR, to see what is proposed here makes any sense ;)

[AndrewKeepCoding]

Hi @niels9001. I guess we should fix #2003 first. 🙂