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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013Gf7WaExcJ9bs3kfJXB3nK

frontend/src/components/Flow/liveStore.ts

@@ -167,6 +167,10 @@ export const liveStore = {
   getFailure(nodeId: string) {
     return failures.get(nodeId)
   },
+  /** Drop the marker for a failure the engine has already been told about. */
+  clearFailure(nodeId: string) {
+    if (failures.delete(nodeId)) notify(`failure:${nodeId}`)
+  },
   /**
    * Dismiss a node's failure, here and on the engine.
    *

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