Prompt step by step for the missing base image, ISO, npm deps and Chromium

Offer to build the base QCOW2, download the ISO, run npm ci and install
Chromium (from the npmmirror mirror by default). Let Playwright locate its
own Chromium so macOS works.

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

build-windows-base.sh

@@ -31,6 +31,8 @@ set -euo pipefail
 
 images_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
 downloader="${images_dir}/download-images.sh"
+# Path lookups below must not offer to run this script again.
+export VM_IMAGE_NO_PROMPT=1
 win_dir="${images_dir}/windows11"
 
 usage() {
@@ -116,6 +118,17 @@ if ((!from_img)); then
     || die "install socat or nc (it presses a key at the boot prompt of the ISO)."
 fi
 
+# Without the ISO, offer to download it (needs a terminal).
+if [[ -z "${WIN11_VM_INSTALL_ISO:-}" ]] && ! "$downloader" --print-path windows11 --iso >/dev/null 2>&1; then
+  "$downloader" --print-path windows11 --iso >&2 || true
+  if { : </dev/tty >/dev/tty; } 2>/dev/null; then
+    printf 'Download the Windows 11 ISO now? [y/N] ' >/dev/tty
+    read -r answer </dev/tty || answer=
+    [[ "$answer" == [yY] || "$answer" == [yY][eE][sS] ]] || exit 1
+    "$downloader" windows11 || exit 1
+  fi
+fi
+
 output="${output:-$("$downloader" --print-path windows11)}"
 if [[ -e "$output" && $force -eq 0 ]]; then
   die "${output} exists (it may be the backing file of existing VM disks); use --force or another OUTPUT."