fix(auth-next,examples): address PR feedback and deduplicate code

- Remove standalone auth-next-default-test, keep wallets-connect-with-nextjs
- Remove unused NEXT_PUBLIC_IMMUTABLE_CLIENT_ID from .env.example
- Document AUTH_SECRET generation in wallets-connect-with-nextjs README
- Document Partial<LoginConfig> design in useLogin/useLogout hooks
- Extract defaultConfig.ts, deduplicate auth-next-server and auth-next-client
- Consolidate constants in auth-next-server, client re-exports
- Add default auth examples to auth-next-client and auth-next-server READMEs

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: rodrigo-fournier-immutable