Centralize retained UI overlay insertion

src/legacy_ui_overlay_services.cpp

@@ -0,0 +1,34 @@
+#include "pch.h"
+#include "legacy_ui_overlay_services.h"
+
+#include "app.h"
+#include "node.h"
+
+namespace pp::panopainter {
+
+void initialize_legacy_overlay_node(App& app, Node& node)
+{
+    node.set_manager(&app.layout);
+    node.init();
+    node.create();
+    node.loaded();
+}
+
+pp::foundation::Status attach_legacy_overlay_node(
+    App& app,
+    const std::shared_ptr<Node>& node) noexcept
+{
+    if (!node) {
+        return pp::foundation::Status::invalid_argument("legacy overlay node is null");
+    }
+
+    auto* root = app.layout[app.main_id];
+    if (!root) {
+        return pp::foundation::Status::invalid_argument("legacy overlay root is missing");
+    }
+
+    root->add_child(node);
+    return pp::foundation::Status::success();
+}
+
+} // namespace pp::panopainter