P0: Add "confirm" phase — wizard checks prereqs then shows Install button,
user must explicitly click before any brew/winget/npm command runs.
P1: Closing the wizard dialog while install is running now auto-calls
install:cancel to stop the backend process.
P2: On Windows, use taskkill /T /F /PID to kill the entire process tree
instead of just the shell wrapper, ensuring npm/winget actually stops.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>