fix: Prevent gamemode slider movement when hardcore mode is enabled

Add virtual `canMoveSlider` method to UIScene base class and override it
in both LoadMenu and CreateWorldMenu scenes. The method returns false
when attempting to move the gamemode slider while hardcore mode is active,
effectively locking the slider in that state.

The UIController now checks `canMoveSlider` before initiating a slider
drag and during ongoing drag updates.

Minecraft.Client/Common/UI/UIScene_LoadMenu.cpp

@@ -1040,6 +1040,14 @@ void UIScene_LoadMenu::handleSliderMove(F64 sliderId, F64 currentValue)
 	}
 }
 
+bool UIScene_LoadMenu::canMoveSlider(F64 sliderId)
+{
+    if (sliderId == eControl_GameMode && m_bHardcore)
+        return false;
+    
+    return true;
+}
+
 void UIScene_LoadMenu::handleTouchBoxRebuild()
 {
 	m_bRebuildTouchBoxes = true;