feat: Refactor editor structure and enhance multi-session support

- Moved `menu_builder.cc` to a new `ui` directory for better organization.
- Updated `EditorManager` to generate unique ImGui IDs for multi-session support, ensuring separate window handling.
- Added session ID management in `EditorContext` for improved child panel identification.
- Introduced a new `MenuBuilder` class for streamlined ImGui menu creation, enhancing UI maintainability.

src/app/editor/editor.h

@@ -28,6 +28,11 @@ struct EditorContext {
   HistoryManager history_manager;
   PopupManager* popup_manager = nullptr;
   ShortcutManager shortcut_manager;
+  
+  // Session identification for multi-session support
+  // Used by child panels to create unique ImGui IDs
+  size_t session_id = 0;
+  
   // Cross-session shared clipboard for editor data transfers
   struct SharedClipboard {
     // Overworld tile16 selection payload