feat(editor): terrain-aligned objects, batch convert, WCP import+load

New features:
- Align to Slope: rotates objects to match terrain surface normal at
  their position (trees on hillsides lean naturally). Works with
  multi-select. Available in object panel and right-click context menu
- Batch Convert Assets: File menu option to recursively convert all
  M2→WOM and WMO→WOB files in a data directory to open format
- Import & Load: one-click WCP unpack + auto-open the imported zone
- sampleTerrainNormal() for slope detection via height differencing
- Zone load error toasts for missing/corrupt files

tools/editor/editor_app.hpp

@@ -89,7 +89,9 @@ public:
     void setGizmoAxis(TransformAxis axis);
     void setSkyPreset(int preset); // 0=day, 1=dusk, 2=night
     void snapSelectedToGround();
+    void alignSelectedToTerrain();
     void flyToSelected();
+    int batchConvertAssets(const std::string& dataDir);
     void clearAllObjects();
     void generateCompleteZone();
     void centerOnTerrain();