feat: remove optional services step from setup wizard

Services are now installed post-setup from the Store page, so the
wizard step that let users pre-select email/calendar/files is removed.
Reduces wizard from 5 steps to 4 (Step4Services deleted, Step5Review
renamed to Step4Review). Backend drops services_enabled validation,
background install thread, and service_store_manager dependency.

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

tests/test_setup_manager.py

@@ -50,7 +50,6 @@ def _valid_payload(**overrides):
         'password': 'SecurePass1!',
         'domain_mode': 'lan',
         'timezone': 'UTC',
-        'services_enabled': ['wireguard'],
         'ddns_provider': 'none',
     }
     base.update(overrides)
@@ -203,7 +202,7 @@ def test_complete_setup_calls_set_identity_field_for_each_field(
         setup_manager.complete_setup(_valid_payload())
     calls = mock_config_manager.set_identity_field.call_args_list
     field_names = [c[0][0] for c in calls]
-    for expected in ('cell_name', 'domain_mode', 'timezone', 'services_enabled', 'ddns_provider'):
+    for expected in ('cell_name', 'domain_mode', 'timezone', 'ddns_provider'):
         assert expected in field_names, f"set_identity_field not called for '{expected}'"