docs: Mini Apps authentication page missing QuickAuth integration example

[CryptoExplor]

Problem

The Mini Apps documentation covers authentication at a high level but does not include a complete end-to-end example of integrating @farcaster/quick-auth for server-side JWT verification.

Developers building mini-apps that need to:

• Authenticate users server-side using their FID
• Protect API routes in Next.js / Hono / Express
• Verify QuickAuth JWTs without a third-party service

...currently have to piece together the flow from the quick-auth README and separate spec pages.

Proposed Addition

Add a dedicated "Authentication" section to the Mini Apps docs with:

1. A step-by-step QuickAuth flow diagram
2. Client-side code: const token = await sdk.quickAuth.fetch('/api/protected')
3. Server-side JWT verification example:

import { createClient } from '@farcaster/quick-auth';

const client = createClient();

export async function GET(req: Request) {
  const token = req.headers.get('Authorization')?.replace('Bearer ', '');
  if (!token) return new Response('Unauthorized', { status: 401 });
  
  const payload = await client.verifyJwt({ token, domain: 'yourdomain.com' });
  const fid = payload.sub; // verified FID as string
  
  return Response.json({ fid });
}

4. Notes on JWT expiry, domain binding, and security considerations

Why This Matters

Authentication is one of the most common needs for mini-app developers building anything beyond read-only UIs. A clear, complete example in the official docs would significantly reduce the barrier to secure mini-app development.