Add SaveSnapshot node with visual separation and rolling limit

Introduce a SaveSnapshot custom node that triggers snapshot captures
via WebSocket. Node-triggered snapshots are visually distinct in the
sidebar (purple left border + "Node" badge) and managed with their
own independent rolling limit (maxNodeSnapshots setting), separate
from auto/manual snapshot pruning. Node snapshots skip hash-dedup
so repeated queue runs always capture.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

__init__.py

@@ -6,7 +6,8 @@ to browse and restore any previous version. Stored in server-side JSON files.
 """
 
 from . import snapshot_routes
+from .snapshot_node import SaveSnapshot
 
 WEB_DIRECTORY = "./js"
-NODE_CLASS_MAPPINGS = {}
-NODE_DISPLAY_NAME_MAPPINGS = {}
+NODE_CLASS_MAPPINGS = {"SaveSnapshot": SaveSnapshot}
+NODE_DISPLAY_NAME_MAPPINGS = {"SaveSnapshot": "Save Snapshot"}