fix(windows): destroy window before quitting message loop

[Davit-Meliqyan]

Summary

Fixes a Windows shutdown delay when setPreventClose(true) is enabled.

Problem

windowManager.destroy() currently calls:

PostQuitMessage(0);

This exits the Win32 message loop before the native window is actually destroyed.

As a result, Flutter Engine shutdown happens outside the normal window lifecycle and the process may remain alive for around 17–18 seconds before exiting.

Solution

Replace:

PostQuitMessage(0);

with:

HWND hWnd = GetMainWindow();
is_prevent_close_ = false;
DestroyWindow(hWnd);

This restores the normal Win32 shutdown sequence:

WM_CLOSE
→ DestroyWindow
→ WM_DESTROY
→ FlutterWindow::OnDestroy()
→ Flutter Engine shutdown
→ PostQuitMessage

Verification

Tested on Windows 11.

Before:

• Closing the application with setPreventClose(true) caused a ~17–18 second delay before process exit.

After:

• Application exits immediately in both Debug and Release builds.