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.hpp

@@ -58,7 +58,13 @@ public:
     core::Window* getWindow() { return window_.get(); }
 
     EditorMode getMode() const { return mode_; }
-    void setMode(EditorMode m) { mode_ = m; }
+    void setMode(EditorMode m) {
+        if (m != mode_) {
+            viewport_.clearGhostPreview();
+            viewport_.setBrushIndicator({}, 0, false);
+        }
+        mode_ = m;
+    }
     void markObjectsDirty() { objectsDirty_ = true; }
 
     void startGizmoMode(TransformMode mode);