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/terrain_editor.hpp

@@ -30,6 +30,9 @@ public:
     // Raycast against terrain, returns true if hit
     bool raycastTerrain(const rendering::Ray& ray, glm::vec3& hitPos) const;
 
+    // Sample terrain normal at a world XY position (for object alignment)
+    glm::vec3 sampleTerrainNormal(const glm::vec3& worldPos) const;
+
     // Apply brush at current position (call per-frame while painting)
     void applyBrush(float deltaTime);