Apple OAuth flow should include nonce to prevent ID token replay

[bpowers]

Description

The Apple OAuth sign-in flow does not include a nonce parameter. While the server performs JWKS-based verification of the Apple ID token (validating signature, issuer, audience, and expiry), it does not bind the token to a specific authentication request via a nonce.

Apple recommends including a nonce -- generated per authentication request and bound to the session/state -- to prevent ID token replay attacks. Without a nonce, a valid ID token intercepted from one authentication flow could theoretically be replayed in another session.

Why it matters

• Security: Prevents ID token replay attacks, following Apple's recommended best practices for Sign in with Apple
• Defense in depth: The existing JWKS verification and short token expiry provide strong baseline security, but nonce binding adds another layer

Components affected

• src/server/ -- OAuth route handlers and token verification logic

Possible approach

1. Generate a cryptographically random nonce during the authorization request
2. Bind the nonce to the OAuth state parameter (which is already session-bound)
3. Pass the nonce to Apple's authorization endpoint
4. After receiving the ID token, verify that the nonce claim in the decoded JWT matches the expected value

Context

Identified during review of the server-side-auth branch (commits implementing server-side OAuth state binding and Apple provider handling).