Add example for integrating Firebase App Check as custom provider

[Copilot]

This PR provides a comprehensive example demonstrating how to integrate Prosopo Procaptcha as a Custom Provider for Firebase App Check in a Nuxt 3 application.

What's Included

1. Custom Provider Backend (demos/firebase-appcheck-example/server/): Express server that validates Procaptcha tokens and issues Firebase App Check tokens
2. Nuxt 3 Client (demos/firebase-appcheck-example/nuxt-client/): Complete Nuxt 3 application with Procaptcha integration and Firebase App Check
3. Firebase Cloud Functions (demos/firebase-appcheck-example/firebase-functions/): Example protected functions that require App Check verification
4. Comprehensive Documentation (demos/firebase-appcheck-example/README.md): Step-by-step guide with architecture diagram, code examples, troubleshooting, and security considerations

Key Features

• Complete end-to-end integration example
• Security best practices documented
• Environment configuration examples
• CORS handling for development
• Error handling and validation
• TypeScript support throughout
• Copyright headers on all code files

The example follows the existing repository structure by placing it in the demos/ directory alongside other examples like client-example-server and client-bundle-example.

Fixes prosopo/captcha#1459

Original prompt

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This section details on the original issue you should resolve

<issue_title>How to implement with Firebase Appcheck</issue_title> <issue_description>I want to integrate this package with Firebase Appcheck as a Custom Provider on a Nuxt3 application. Are there any examples I could follow to achieve this?</issue_description>

<agent_instructions>Please provide an example of how to integrate with Firebase Appcheck</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@forgetso Hi @andrevferreiraa, we don't have examples of this but will endeavour to take a look and update you soon.

• Fixes prosopo/captcha#1459

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[HughParry]

@copilot continue work

[Copilot]

@copilot continue work

The Firebase App Check integration example is complete and ready for review. The example includes:

• Custom provider backend for token verification
• Nuxt 3 client with Procaptcha integration
• Firebase Cloud Functions examples
• Comprehensive documentation with step-by-step instructions

All files are in demos/firebase-appcheck-example/ and follow the repository structure. The example has passed code review and security checks.