Refactor constants initialization

examples/password_manager_demo.py

@@ -1,9 +1,11 @@
 from password_manager.manager import PasswordManager
 from nostr.client import NostrClient
+from constants import initialize_app
 
 
 def main() -> None:
     """Show how to initialise PasswordManager with Nostr support."""
+    initialize_app()
     manager = PasswordManager()
     manager.nostr_client = NostrClient(encryption_manager=manager.encryption_manager)
     # Sample actions could be called on ``manager`` here.