Add websocket node: client or served socket, listener or sender

ws:// and wss:// URLs dial out and hold a supervised connection; a path is
served under /hooks/<flow>/ with an optional secret. Outputs only listen,
inputs send. Frames are JSON keyed by port, with json_key and on_connect.

frontend/src/components/Flow/FlowNode.tsx

@@ -4,6 +4,7 @@ import {
   Bird,
   Braces,
   Bug,
+  Cable,
   Clock,
   Code2,
   Database,
@@ -45,6 +46,7 @@ const NODE_ICONS = {
   python: Code2,
   mqtt: Radio,
   http: Globe,
+  websocket: Cable,
   influxdb: Database,
   delay: Clock,
   mlp: Braces,