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.

tidal_dl_ng/gui/dialog_playlist_manager.py

@@ -316,8 +316,7 @@ class PlaylistManagerDialog(QtWidgets.QDialog):
 
         TODO: Integrate with app's notification system (Toast/Snackbar)
         """
-        # Silent - errors are shown via UI notifications
-        pass
+        logger_gui.debug(f"PlaylistManagerDialog notification: {message}")
 
     def closeEvent(self, event: QtGui.QCloseEvent) -> None:
         """Handle dialog close event.