build-windows-base.sh: show a window by default, fix macOS bash 3.2 and fork crashes

Replace --window with --headless (VNC); the window is the default. Fill the
QEMU config placeholders without nested quotes (bash 3.2 kept them literally),
and run QEMU as a background job instead of -daemonize when it shows a window
(Cocoa crashes in fork()).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -130,7 +130,7 @@ It needs `qemu`, `qemu-img`, an ISO tool (`xorriso`, `genisoimage` or `mkisofs`)
 and `socat` or `nc`; there is no SSH to the guest.
 
 ```bash
-./build-windows-base.sh [--force] [--compress] [--from-img] [--window] [OUTPUT.qcow2]
+./build-windows-base.sh [--force] [--compress] [--from-img] [--headless] [OUTPUT.qcow2]
 ```
 
 It refuses to replace an existing base image without `--force`, because the