feat(cli): add Auth.js and shadcn/ui plugins

[gonzoblasco]

Add Auth.js and shadcn/ui (#268)

Closes #268

Summary

Adds two new plugins: Auth.js (NextAuth v5) for authentication and shadcn/ui for accessible component library support.

Auth.js plugin

• Dependency: next-auth@beta (v5)
• Generated files:
  • auth.ts — NextAuth config with empty providers array
  • app/api/auth/[...nextauth]/route.ts — route handler exporting GET/POST
  • proxy.ts — Next.js 16 middleware (session refresh)
• Environment variable: AUTH_SECRET
• Validation: requires App Router (--router=app)
• Flag: --auth-js

shadcn/ui plugin

• Dependencies: class-variance-authority, clsx, tailwind-merge, tailwindcss-animate, lucide-react
• Dev dependency: shadcn (for CLI usage)
• Generated files:
  • components.json — shadcn config (new-york style, neutral base color, CSS variables enabled)
  • lib/utils.ts — cn() helper using clsx + tailwind-merge
• Script: ui:add (shadcn add) for adding components
• Validation: requires Tailwind CSS (--styling=tailwind-css)
• Flag: --shadcn

Design decisions

• Two separate plugins: Auth.js and shadcn/ui can be used independently. The issue asks for both, but they don't depend on each other.
• Auth.js v5 (beta): This is the current version with native App Router support. The proxy.ts file follows the Next.js 16 naming convention (formerly middleware.ts).
• shadcn/ui manual file generation: Instead of running shadcn init (which is interactive), the plugin generates components.json and lib/utils.ts directly. Users can then run shadcn add <component> to add components as needed.
• No components pre-installed: The plugin sets up the infrastructure but doesn't add any shadcn components. Users choose what they need via shadcn add.

Verification

• ✅ pnpm run build — clean
• ✅ pnpm run lint — no errors
• ✅ pnpm run unit — 9/9 tests pass
• ✅ ./bin/run --help shows --auth-js and --shadcn flags
• ✅ Validation works: --auth-js --router=pages throws error
• ✅ Validation works: --shadcn --styling=css-modules throws error

Notes

• Website (create-next-stack.com) is NOT updated in this PR.
• Auth.js requires App Router because v5 is designed for App Router. Pages Router support was dropped in v5.
• shadcn/ui requires Tailwind CSS because it's built on Tailwind.

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