Sets up shadcn/ui CLI so new components can be added with
`npx shadcn add <component>`. Uses new-york style, zinc base color,
no CSS variables (matches existing Tailwind-only approach).
Adds clsx + tailwind-merge for the cn() utility.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
