Cancellable window close request (and app quit request) event

Start by tracing the platform close paths named in the issue: WM_QUERYENDSESSION/WM_ENDSESSION, WM_CLOSE, macOS Cmd+Q, GTK delete-event, and Window::Close(). Review the linked window_manager reports, especially the AppLifecycleListener.onExitRequested compatibility concern. Done means defining and implementing a cross-platform cancellable window-close event and application-level quit request without swallowing system shutdown or breaking Flutter lifecycle handling.

There's no way to intercept a window being closed. Only WindowClosedEvent fires after the fact. window_manager 0.6.0 currently can't implement setPreventClose / onWindowClose for the system close button, and documents that as a gap.

Proposal: a cancellable close-request event per window, raised by the close button, Alt+F4, the taskbar/Dock menu and delete-event, plus Window::Close(). Requirements from the reports below:

• system shutdown, logoff and installer-initiated closes (WM_QUERYENDSESSION/WM_ENDSESSION, or a WM_CLOSE from another process) must not be silently swallowed
• Cmd+Q / app termination on macOS should surface as an application-level quit request (cancellable), distinct from window close
• it must not break Flutter's own AppLifecycleListener.onExitRequested when nobody listens
• possibly the same pattern for minimize/maximize requests

Background:

• leanflutter/window_manager#356: setPreventClose blocks the MSI installer from closing the app
• leanflutter/window_manager#362: Cmd+Q doesn't trigger onWindowClose on macOS
• leanflutter/window_manager#466: window_manager breaks AppLifecycleListener.onExitRequested
• leanflutter/window_manager#316: need setPreventMinimize / setPreventMaximize