Announce an acknowledged node failure on the bus

Clearing a node's last error on the engine published nothing, so a second
browser kept the marker until its next snapshot. One event carries the
qualified node; the receiving client drops the marker without refetching.

frontend/src/components/Flow/useFlowSocket.ts

@@ -57,6 +57,7 @@ type FlowEvent =
       ts?: number
     }
   | { type: "node_status"; node: string; status: string; error?: string | null }
+  | { type: "node_error_acknowledged"; node: string; ts?: number }
   | ({ type: "node_log" } & LogLine)
   | { type: "flow_paused"; flow: string; paused: boolean }
   | {
@@ -235,6 +236,11 @@ function connect() {
           error: message.error,
         })
         break
+      case "node_error_acknowledged":
+        // Someone dismissed it, here or in another browser. The engine has
+        // already forgotten it, so drop the marker rather than posting back.
+        liveStore.clearFailure(message.node)
+        break
       case "node_log":
         liveStore.appendLog(message)
         break