fix(registry): store nodes per-client to prevent multi-tab race condition

Move NodeRegistry from a single global _nodes dict to a per-client
(_tab_nodes) structure so that multiple ComfyUI browser tabs no
longer overwrite each other's workflow node data during a
lora_registry_refresh cycle.  The merged result is a union of all
known tabs' target nodes, eliminating the non-deterministic failure
where send-to-workflow could randomly target a tab lacking valid
targets.

- NodeRegistry.register_nodes(sid, nodes) replaces per-tab data
  without affecting other tabs.
- NodeRegistry.get_merged_registry() returns the union across all
  connected clients, together with tab_count / per-tab metadata.
- prepare_for_refresh() snapshots the current active sockets; caller
  re-reads before merging so that newly-connected tabs are not pruned.
- workflow_registry.js sends api.clientId in the POST body so the
  backend can identify which tab is registering.

web/comfyui/workflow_registry.js

@@ -151,7 +151,10 @@ app.registerExtension({
                 headers: {
                     "Content-Type": "application/json",
                 },
-                body: JSON.stringify({ nodes: workflowNodes }),
+                body: JSON.stringify({
+                    nodes: workflowNodes,
+                    client_id: api.clientId ?? api.initialClientId ?? "",
+                }),
             });
 
             if (!response.ok) {