Restore enterprise/LDAP login mode detection

- Added /users/auth-mode public endpoint to detect LDAP status
- Updated frontend to call getAuthMode() instead of hardcoded false
- Login page now automatically switches to enterprise mode when LDAP is enabled
- Fixes missing LDAP user login UI that was previously disabled with TODO comment

This restores the functionality that was commented out earlier where the login
page would auto-detect whether to show local user list or enterprise username input.

backend/routers/auth.py

@@ -257,6 +257,16 @@ def login(request: Request, form_data: schemas.UserLogin, db: Session = Depends(
     )
 
 
+@router.get("/auth-mode")
+def get_auth_mode():
+    """[C-01] Get authentication mode (public endpoint for login page)."""
+    config = get_ldap_config()
+    return {
+        "mode": "enterprise" if config.get("ldap_enabled") else "local",
+        "ldap_enabled": config.get("ldap_enabled", False)
+    }
+
+
 @router.get("/ldap-config")
 def get_ldap_settings(current_user: auth.TokenData = Depends(auth.get_current_admin)):
     """[C-01] Get LDAP config — admin only."""