feat: Enhance playlist management and GUI logic

- Improved playlist item fetching by adding robust pagination and low-level request hooks for better testing and flexibility.
- Refactored track ID extraction logic into reusable helpers for clarity.
- Improved initialization for dialog components to avoid circular dependencies in the GUI.
- Added new unit tests to validate playlist operations, including track addition/removal and API integration.
- Updated metadata functions to simplify dictionary access using `.get` for better readability.
- Ensured PySide6 system dependencies are installed in GitHub Actions to address CI compatibility issues.
- Streamlined UI package initialization to improve modularity and robustness of imports.

tidal_dl_ng/ui/__init__.py

@@ -0,0 +1,37 @@
+"""UI package initializer.
+
+- Ensures that `tidal_dl_ng.ui.dialog_playlist_manager` exposes both the generated
+  `Ui_DialogPlaylistManager` and the implementation `PlaylistManagerDialog`.
+- Does not modify generated UI .py files; composes at import time.
+"""
+
+from __future__ import annotations
+
+import importlib
+from types import ModuleType
+
+# Import generated UI module
+_ui_mod: ModuleType = importlib.import_module("tidal_dl_ng.ui.dialog_playlist_manager")
+
+# Attach the implementation class from gui
+try:
+    from tidal_dl_ng.gui.dialog_playlist_manager import PlaylistManagerDialog as _ImplDialog
+except Exception:
+    _ImplDialog = None  # type: ignore[assignment]
+
+if _ImplDialog is not None:
+    _ui_mod.PlaylistManagerDialog = _ImplDialog
+
+# Re-export for convenience when importing from the package
+if _ImplDialog is not None:
+    PlaylistManagerDialog = _ImplDialog  # type: ignore[assignment]
+
+try:
+    from tidal_dl_ng.ui.dialog_playlist_manager import Ui_DialogPlaylistManager as _UiClass
+except Exception:
+    _UiClass = None  # type: ignore[assignment]
+
+if _UiClass is not None:
+    Ui_DialogPlaylistManager = _UiClass  # type: ignore[assignment]
+
+__all__ = [name for name in ("PlaylistManagerDialog", "Ui_DialogPlaylistManager") if name in globals()]