feat: add passkeys support for web

[NandanPrabhu]

Summary

• Implement passkeySignupChallenge, passkeyLoginChallenge, and getTokenByPasskey on the web platform by delegating to @auth0/auth0-spa-js's passkey API (getSignupChallenge, getLoginChallenge, and the internal _requestTokenForPasskey token exchange), replacing the previous UnsupportedOperation stubs.
• Bump @auth0/auth0-spa-js 2.19.3 → 2.24.0 to pick up its passkey support.
• Extend PasskeyError's error-code map to normalize spa-js's passkey error codes into the existing PasskeyErrorCodes.
• Keep the public contract identical to native (authParamsPublicKey object, authResponse as a JSON string) so app code stays cross-platform — only the credential-manager step differs (browser navigator.credentials vs. a native module).
• Update README.md, EXAMPLES.md, EXAMPLES-WEB.md, and CLAUDE.md to reflect web support, including a web-specific example showing the navigator.credentials serialization step.
• Wire up a working Passkeys section in the web example app (example/src/App.web.tsx).

Test plan

• yarn test — 35 suites / 607 tests passing, including new WebAuth0Client.spec.ts coverage for passkey challenge mapping, token exchange, and error propagation
• yarn typecheck / yarn lint clean
• Manually verified end-to-end in a browser against a mocked Auth0 tenant (RS256-signed ID tokens, JWKS, virtual WebAuthn authenticator): signup and login both complete and the app renders the authenticated user

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• Create PR with unit tests
• Commit unit tests in branch feat/passkeys-web

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