style: Introduce Singleton pattern for operations

Apply Singleton pattern to the `Operations` class to ensure a single instance across the application. This enforces a consistent state and simplifies interaction with shared resources.

operations.py

@@ -8,6 +8,7 @@
 
 from gui import SIMPLENOTE_BASE_DIR, SIMPLENOTE_SETTINGS_FILE, _show_message, open_view, remove_status
 from models import Note
+
 from .utils.patterns.singleton.base import Singleton