feat(main_menu): add exit button and refactor layout with ColumnLayout

- Add `should_close_window()` method to force window close from UI.
- Replace standalone button with ColumnLayout for proper centering.
- Add "Exit" button that calls `should_close_window()`.
- Update ESC key handling to only close window when game is not running.
- Fix ColumnLayout to respect parent's anchor instead of hardcoded TOP_LEFT.
- Remove unused parameter name in Image::update for consistency.

src/ui/column_layout.cpp

@@ -16,7 +16,7 @@ void ColumnLayout::layout() {
     int y = 0;
     for (auto& child : children) {
         child->set_offset({0, y});
-        child->set_anchor(Anchor::TOP_LEFT);
+        child->set_anchor(m_anchor);
         y += child->height() + m_spacing;
     }
 }