Strengthen setup password validation

- Add backend Pydantic password complexity validation for setup
- Update frontend setup page with password rule feedback and strength indicator
- Add/adjust setup API tests for password validation
- Document setup password requirements
- Fix frontend test type annotation issue

backend/tests/test_main.py

@@ -298,7 +298,7 @@ async def test_startup_overrides_settings_from_persisted_setup(tmp_path: Path) -
     await init_db(db)
     await setup_service.run_setup(
         db,
-        master_password="supersecret123",
+        master_password="Supersecret1!",
         database_path=runtime_db_path,
         fail2ban_socket="/tmp/persisted.sock",
         timezone="Europe/Berlin",