Add Next.js App Router example

[ImBIOS]

Description

This PR adds a new example application using Next.js App Router. It demonstrates how to use Hanko authentication with the newer App Router architecture in Next.js.

Changes made:

• Created a new Next.js App Router example in /frontend/examples/nextjs-app/
• Updated the main README to list the new example
• Created a Docker Compose configuration for the App Router example in deploy/docker-compose/todo-nextjs-app.yaml

Features:

• Uses the modern App Router architecture from Next.js
• 'use client' directives for client components
• Same functionality as the Pages Router example, but with updated architecture
• Simplified routing and layout structure

Testing:

The app can be tested by running:

docker compose -f deploy/docker-compose/base.yaml -f deploy/docker-compose/todo-nextjs-app.yaml -p "hanko-todo-nextjs-app" up --build