Fix AttributeError: remove redundant mapping_learned connection in main.py

TopBarWidget already connects mapping_learned to _on_mapping_learned internally
in set_midi_engine, so the external connection in main.py was both redundant
and referenced a non-existent method.

cart_widget.py

@@ -18,7 +18,6 @@ class CartSlot(QFrame):
         self.slot_number = slot_number
         self.engine      = engine
         self._playing_icon = False
-        self._playing_icon = False
 
         self.setFrameStyle(QFrame.Box | QFrame.Raised)
         self.setLineWidth(1)
@@ -146,7 +145,6 @@ class CartSlot(QFrame):
         root.addLayout(btn_col)
 
     def apply_theme(self, theme: dict):
-        """Apply theme to cart slot LCD."""
         self.lcd_frame.setStyleSheet(f"""
             QFrame {{
                 background-color: {theme['bg']};
@@ -291,7 +289,5 @@ class CartBankWidget(QWidget):
         root.addLayout(grid)
 
     def apply_theme(self, theme: dict):
-        """Propagate theme to all cart slots."""
         for slot in self.slots:
-            slot.apply_theme(theme)
-            
+            slot.apply_theme(theme)