feat(editor): WCP shortcut, help panel overhaul, active path restore

- Ctrl+Shift+E keyboard shortcut for WCP content pack export
- Help panel expanded: object tools (snap/align/flatten/scatter/select
  by type), all terrain generators, stamp save/load, export shortcuts
- ObjectPlacer::loadFromFile restores activePath_ from loaded objects
  so users can continue placing the same type after loading
- WCP export menu item now shows Ctrl+Shift+E hint

tools/editor/object_placer.cpp

@@ -270,6 +270,12 @@ bool ObjectPlacer::loadFromFile(const std::string& path) {
             }
         }
 
+        // Restore active path from last loaded object for seamless placement
+        if (!objects_.empty()) {
+            activePath_ = objects_.back().path;
+            activeType_ = objects_.back().type;
+        }
+
         LOG_INFO("Objects loaded: ", path, " (", objects_.size(), " objects)");
         return true;
     } catch (const std::exception& e) {