fix preview and save on native

tests/test_workflow_save.py

@@ -0,0 +1,20 @@
+from pathlib import Path
+
+import pytest
+
+from backend.server import PNG_SIGNATURE, save_png_bytes
+
+
+def test_save_png_bytes_writes_exact_png_payload(tmp_path: Path):
+    target = tmp_path / "workflow"
+    payload = PNG_SIGNATURE + b"argonode-test-payload"
+
+    saved_path = save_png_bytes(str(target), payload)
+
+    assert saved_path == tmp_path / "workflow.png"
+    assert saved_path.read_bytes() == payload
+
+
+def test_save_png_bytes_rejects_invalid_payload(tmp_path: Path):
+    with pytest.raises(ValueError, match="valid PNG"):
+        save_png_bytes(str(tmp_path / "workflow.png"), b"not-a-png")