fix(ui): QtSignal → Signal (PySide6兼容)

src/ui/main_window.py

@@ -8,7 +8,7 @@ from PySide6.QtWidgets import (
     QStackedWidget, QStatusBar, QMenuBar, QMenu,
     QLabel, QFrame, QSplitter, QSizePolicy
 )
-from PySide6.QtCore import Qt, QSize
+from PySide6.QtCore import Qt, QSize, Signal
 from PySide6.QtGui import QAction, QIcon
 
 from .theme import ThemeColors, ThemeQSS
@@ -70,7 +70,7 @@ class TitleBar(QWidget):
 class NavigationBar(QWidget):
     """Left side navigation bar."""
     
-    navigation_changed = QtSignal(str)
+    navigation_changed = Signal(str)
     
     def __init__(self, parent=None):
         super().__init__(parent)