fix missing Signal import in scanner_tab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gui/src/tabs/scanner_tab.py

@@ -7,7 +7,7 @@ from __future__ import annotations
 
 import json
 
-from PySide6.QtCore import Qt, QTimer
+from PySide6.QtCore import Qt, QTimer, Signal
 from PySide6.QtGui import QFont, QColor
 from PySide6.QtWidgets import (
     QWidget, QSplitter, QVBoxLayout, QHBoxLayout,