SpacerNode: default positioningMode, aspectRatioLock and variables

SpacerNode.fromJson required positioningMode and aspectRatioLock, which
every other node defaults, so a spacer without them failed to parse.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012anUJ6o6K6kjUXur8Ak8XX

Author: claude

lib/src/api/nodes/spacer_node.dart

@@ -41,6 +41,10 @@ class SpacerNode extends SceneNode {
     super.edgePins,
     super.parentID,
     super.constraints,
+    super.positioningMode,
+    super.aspectRatioLock,
+    super.variables,
+    super.multipleVariables,
   });
 
   /// Creates a new [SpacerNode] from a JSON data.