Add multi-user auth with sessions and login UI

Add multi-user auth with sessions and login UI

- auth: POST /auth validates PAM, returns session token
- auth: GET /me validates Bearer token, returns username
- auth: POST /logout destroys session
- auth: POST /users creates Linux user via useradd+chpasswd
- session.c: in-memory token store (1024 slots, pthread mutex, /dev/urandom tokens)
- user.c: fork/exec useradd + chpasswd with username validation
- ui: login overlay shown until authenticated (x-cloak)
- ui: token persisted in sessionStorage, validated on page load
- ui: username displayed in dock after login
- ui: apps only load after successful auth

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>