Implements #750 - allows hiding the username/password login form when
using OIDC SSO to avoid user confusion, while maintaining security.
- Added HideLocalLogin config option (env: PULSE_AUTH_HIDE_LOCAL_LOGIN)
- Exposed hideLocalLogin in /api/security/status endpoint
- Updated Login.tsx to conditionally hide local login form
- Added escape hatch via ?show_local=true URL parameter
This approach avoids the security and upgrade issues that led to
DISABLE_AUTH being removed (see #707, #678), while solving the UX
problem of users being confused by multiple login options.