feat(ui): introduce ColumnLayout widget and refactor widget parenting

- Add ColumnLayout widget that arranges children vertically with spacing.
- Refactor Widget::add_child to automatically pass `this` as parent.
- Update DebugCollector to use ColumnLayout for consistent spacing.
- Expose children() accessor in Widget for layout management.

src/CMakeLists.txt

@@ -67,4 +67,5 @@ target_sources(${PROJECT_NAME}
         ui/world_ui_manager.cpp
         ui/main_menu_ui_manager.cpp
         scene/main_menu_scene.cpp
+        ui/column_layout.cpp
 )