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.

pyproject.toml

@@ -233,3 +233,9 @@ warn_unused_configs = true
 warn_unused_ignores = true
 disallow_untyped_defs = true
 disallow_any_unimported = true
+
+[tool.deptry]
+# shiboken6 is a transitive dependency from PySide6 that we intentionally use directly
+# for type checking and widget deletion (Shiboken.isValid, Shiboken.delete)
+extend_exclude = []
+ignore_transitive = ["shiboken6"]