rework web server so multiple clients can be server at a time

backend/nodes/save.py

@@ -7,6 +7,7 @@ from pathlib import Path
 import numpy as np
 
 from backend.node_registry import register_node
+from backend.execution_context import emit_warning
 from backend.data_types import DataField, LineData, MeshModel, datafield_to_uint8, image_to_uint8
 
 
@@ -255,7 +256,4 @@ class Save:
         path.write_text("\n".join(lines) + "\n", encoding="utf-8")
 
     def _send_warning(self, message: str):
-        fn = Save._broadcast_warning_fn
-        nid = Save._current_node_id
-        if fn and nid:
-            fn(nid, message)
+        emit_warning(message)