fix(editor): object ID reset on load, zone name validation

- ObjectPlacer::loadFromFile() now resets uniqueIdCounter_ and clears
  selectedIndices_ to prevent ID collisions on repeated loads
- Zone rename validates against path traversal and special characters
  (rejects slashes, dots, colons, control chars, empty strings)
- UI shows error toast for invalid zone names

tools/editor/object_placer.cpp

@@ -244,6 +244,8 @@ bool ObjectPlacer::loadFromFile(const std::string& path) {
         objects_.clear();
         undoStack_.clear();
         selectedIdx_ = -1;
+        selectedIndices_.clear();
+        uniqueIdCounter_ = 1;
 
         for (const auto& jo : arr) {
             PlacedObject obj;