main_window: restore Path import (used at line 69)

Erroneously removed in a51c9a1 — Path is used in __init__ for
set_library_dir(Path(lib_dir)). The dead-code scan missed it.

booru_viewer/gui/main_window.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 import asyncio
 import logging
 import threading
+from pathlib import Path
 
 from PySide6.QtCore import Qt, QTimer, Signal, QUrl
 from PySide6.QtGui import QPixmap, QAction, QKeySequence, QDesktopServices, QShortcut