fix: 500 on setup complete + wizard shows all 7 steps

Two bugs:

1. AttributeError: AuthManager.update_password does not exist — the
   fallback when create_user fails should call set_password_admin().
   This caused a 500 on every setup submit when an admin user already
   existed (e.g. from a previous install attempt).

2. Wizard was jumping to step 2 and skipping domain steps 3-4 when
   preconfigured data existed in cell_config.json. Since the installer
   no longer sets that data, and the wizard must always show all steps,
   the installerConfigured state and all step-skipping navigation is
   removed. Values are still pre-filled if found in config.

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

tests/test_setup_manager.py

@@ -252,7 +252,7 @@ def test_complete_setup_returns_error_when_create_user_fails(
         setup_manager, mock_config_manager, mock_auth_manager, tmp_path):
     mock_config_manager.get_identity.return_value = {}
     mock_auth_manager.create_user.return_value = False
-    mock_auth_manager.update_password.return_value = False
+    mock_auth_manager.set_password_admin.return_value = False
     with patch.dict(os.environ, {'DATA_DIR': str(tmp_path)}):
         result = setup_manager.complete_setup(_valid_payload())
     assert result['success'] is False