fix(editor): normal recalculation after sculpt, mode switch cleanup

- Terrain normals recalculated after height changes (smooth lighting
  on sculpted terrain instead of flat-shaded appearance)
- Ghost preview and brush indicator cleared when switching modes
  (prevents stale model instances or circles persisting)
- File > Clear All resets undo history and selections
- Normal computation uses finite differences from neighbor heights,
  handles both outer (9x9) and inner (8x8) vertex grid positions

tools/editor/editor_app.cpp

@@ -469,6 +469,9 @@ void EditorApp::refreshDirtyChunks() {
     auto dirty = terrainEditor_.consumeDirtyChunks();
     if (dirty.empty()) return;
 
+    // Recalculate normals for modified chunks (better lighting)
+    terrainEditor_.recalcNormals(dirty);
+
     // Regenerate full mesh and reload terrain
     auto mesh = terrainEditor_.regenerateMesh();
     viewport_.clearTerrain();