feat(store,ui): handle Hub error messages and display error banner

- Handle `action: "error"` messages in connection-store (e.g. UNAUTHORIZED)
- Widen lastError type to `{code, message}` to support all error codes
- Display dismissible error banner in Chat with role="alert" and aria-live
- Add accessible close button with focus-visible ring

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/store/src/connection-store.ts

@@ -20,7 +20,6 @@ import {
   GatewayClient,
   StreamAction,
   type ConnectionState,
-  type SendErrorResponse,
   type StreamPayload,
   type AgentEvent,
   type GetAgentMessagesResult,
@@ -35,7 +34,7 @@ interface ConnectionStoreState {
   hubId: string | null
   agentId: string | null
   connectionState: ConnectionState
-  lastError: SendErrorResponse | null
+  lastError: { code: string; message: string } | null
 }
 
 interface ConnectionStoreActions {
@@ -146,13 +145,20 @@ function createClient(
         return
       }
 
+      // Handle error messages from Hub (e.g. UNAUTHORIZED)
+      if (msg.action === "error") {
+        const payload = msg.payload as { code: string; message: string }
+        set({ lastError: { code: payload.code, message: payload.message } })
+        return
+      }
+
       // Handle direct (non-streaming) messages
       const payload = msg.payload as { agentId?: string; content?: string }
       if (payload?.agentId && payload?.content) {
         useMessagesStore.getState().addAssistantMessage(payload.content, payload.agentId)
       }
     })
-    .onSendError((error) => set({ lastError: error }))
+    .onSendError((error) => set({ lastError: { code: error.code, message: error.error } }))
 }
 
 /** Fetch message history from Hub via RPC after connection is established */

packages/ui/src/components/chat.tsx

@@ -16,6 +16,7 @@ export function Chat() {
   const agentId = useConnectionStore((s) => s.agentId)
   const gwState = useConnectionStore((s) => s.connectionState)
   const hubId = useConnectionStore((s) => s.hubId)
+  const lastError = useConnectionStore((s) => s.lastError)
 
   const messages = useMessagesStore((s) => s.messages)
   const streamingIds = useMessagesStore((s) => s.streamingIds)
@@ -65,6 +66,23 @@ export function Chat() {
         )}
       </main>
 
+      {/* Error banner */}
+      {lastError && (
+        <div className="px-4 py-2 max-w-4xl mx-auto w-full" role="alert" aria-live="polite">
+          <div className="rounded-md bg-destructive/10 text-destructive text-sm px-3 py-2 flex items-center justify-between">
+            <span>{lastError.message} ({lastError.code})</span>
+            <button
+              type="button"
+              aria-label="Dismiss error"
+              onClick={() => useConnectionStore.setState({ lastError: null })}
+              className="text-destructive/60 hover:text-destructive ml-2 text-xs focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-1 rounded outline-none"
+            >
+              &times;
+            </button>
+          </div>
+        </div>
+      )}
+
       {/* Footer */}
       <footer className="w-full p-2 pt-1 max-w-4xl mx-auto">
         <ChatInput