feat(nextjs): add Next.js App Router support

[perasperaactual]

Problem

Stackwright's current registration pattern targets Next.js Pages Router (pages/_app.tsx). The Next.js community has largely migrated to App Router (Next.js 13+). New users starting a project today default to App Router and find no supported Stackwright path.

What needs to change

Registration

• Document and test registerNextJSComponents() in app/layout.tsx
• Add 'use client' directives where required

Static generation

• getStaticPropsForSlug / getStaticPaths are Pages Router APIs
• Add App Router equivalents: generateStackwrightStaticParams() and an async page data fetcher

Scaffolding

• Scaffold command (see fix(cli): update scaffold command to generate current project structure #77) should default to App Router
• Keep Pages Router as a documented alternative

Testing

• Verify the prebuild pipeline output is consumed correctly under App Router data fetching model
• Add or migrate an example to App Router

Acceptance criteria

• A working Stackwright site can be built with Next.js App Router
• Registration, static generation, and routing all work
• App Router path is the default in documentation and scaffold output