chore: ruff autofix — import sorting, remove unused imports

Mechanical cleanup via `ruff check --fix` + `ruff format`:
- cli.py, test_auth.py: import sorting (isort convention)
- cli.py: remove unused AuthenticationError import in _run_setup
- config.py: remove unused `field` import
- test_auth.py: remove unused MagicMock import
- test_config.py: remove unused Path import

No functional change. All 131 tests remain green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_config.py

@@ -1,14 +1,14 @@
 """Tests for config.py."""
 
+
 import pytest
 import yaml
-from pathlib import Path
 
 from mcp_synology_container.config import (
     AppConfig,
     ConnectionConfig,
-    _validate_config,
     _merge_env_overrides,
+    _validate_config,
     load_config,
     save_config,
 )