feat: Refactor playlist management, error handling, and testing infrastructure

- Improved playlist item handling with pagination, error logging, and dedicated request hooks.
- Consolidated reusable helpers for track ID normalization, playlist validation, and removal logic.
- Enhanced error handling by introducing `PlaylistNotFound` and `UserNotAuthenticated` exceptions.
- Refined Pytest `qapp` fixture for headless CI compatibility and session-scoped reuse.
- Replaced redundant fixtures in multiple test files for consistency.
- Improved playlist management GUI with detailed debug logging for notifications and operations.
- Added `deptry` configuration to handle transitive dependencies and ignored exclusions.
- Updated Pytest and Tox configurations for consistent environment setup during CI/CD runs.

tests/test_settings_ui.py

@@ -10,16 +10,6 @@ from tidal_dl_ng.dialog import DialogPreferences
 from tidal_dl_ng.ui.dialog_settings import Ui_DialogSettings
 
 
-@pytest.fixture
-def qapp():
-    """Create QApplication instance for tests."""
-    app = QtWidgets.QApplication.instance()
-    if app is None:
-        app = QtWidgets.QApplication([])
-    yield app
-    # Cleanup is handled by pytest-qt if available, otherwise by QApplication
-
-
 class TestUiDialogSettings:
     """Test the generated UI class."""