Add authentication methods to client.go:
- Authenticate(ctx) for explicit login
- AuthenticateWith2FA(ctx, token) for 2FA support
- refreshToken(ctx) for token renewal
- ensureAuthenticated(ctx) for auto-auth before requests
- CurrentUser(ctx) to get logged-in user info
- getToken() for thread-safe token access
Features:
- Token stored with expiry time (23h for safety margin)
- Auto-refresh when token expires within 1 hour
- Falls back to full authentication if refresh fails
- Thread-safe token access using RWMutex
- Sends token via X-Client-Token header
API endpoints used:
- POST /auth/login.json?version=2
- POST /auth/refresh_token.json?version=2
- GET /auth/curr_user.json
Closes checkvist-api-lpn