fix: strip null synth_engine from project JSON

- Frontend serialize: use undefined instead of null for synth_engine
- Backend validate: strip None synth_engine from tracks before schema validation
- Prevents 'None is not of type object' schema error

app/api/v1/projects.py

@@ -99,6 +99,14 @@ def validate_project_data(data_json: str) -> str:
         data = json.loads(data_json)
         if "project_id" not in data:
             data["project_id"] = "temp_legacy_" + str(int(time.time()))
+        # Strip null synth_engine from tracks (breaks schema validation)
+        for session_key in ["main_session"] + [k for k in data.get("section_store", {})]:
+            session = data.get(session_key)
+            if not session:
+                continue
+            for track in session.get("tracks", []):
+                if "synth_engine" in track and track["synth_engine"] is None:
+                    del track["synth_engine"]
         if "main_session" not in data:
             data = upgrade_project_json_if_needed(data)
             data_json = json.dumps(data)