fix(editor): About dialog now works, moved outside menu bar scope

- About dialog was broken because ImGui::OpenPopup inside BeginMenu
  has wrong ID scope — popup never appeared
- Changed to a proper ImGui::Begin window with showAbout_ flag
- Closeable with X button or clicking About again
- Shows version, author, feature summary, and tech stack

tools/editor/editor_ui.hpp

@@ -43,6 +43,7 @@ private:
     bool showLoadDialog_ = false;
     bool showSaveDialog_ = false;
     bool showHelp_ = false;
+    bool showAbout_ = false;
 
     char newMapNameBuf_[256] = "CustomZone";
     int newTileX_ = 32;