fix: Add missing QCheckBox import to overlay_window.py

BUG: name 'QCheckBox' is not defined errors in plugins settings tab.

Root cause: QCheckBox was imported inside _open_settings() method but
_create_plugins_settings_tab() also uses it. The local import didn't
propagate to the helper method.

FIX:
- Added QCheckBox to top-level PyQt6 imports
- Added QTabWidget to top-level imports (also used)
- Removed redundant local imports from _open_settings()

All settings UI components now available globally in the module.

core/overlay_window.py

@@ -18,7 +18,7 @@ try:
         QMainWindow, QWidget, QVBoxLayout, QHBoxLayout,
         QLabel, QPushButton, QStackedWidget, QSystemTrayIcon,
         QMenu, QApplication, QFrame, QListWidget, QListWidgetItem,
-        QButtonGroup, QSplitter, QSizePolicy
+        QButtonGroup, QSplitter, QSizePolicy, QCheckBox, QTabWidget
     )
     from PyQt6.QtCore import Qt, QTimer, pyqtSignal, QSize, QPropertyAnimation, QEasingCurve, QParallelAnimationGroup
     from PyQt6.QtGui import QAction, QIcon, QColor, QFont, QKeySequence, QShortcut
@@ -696,7 +696,7 @@ class OverlayWindow(QMainWindow):
     
     def _open_settings(self):
         """Open settings dialog."""
-        from PyQt6.QtWidgets import QDialog, QVBoxLayout, QTabWidget, QCheckBox, QHBoxLayout
+        from PyQt6.QtWidgets import QDialog
         
         c = get_all_colors()