feat(agent-office): WS reconnect exponential backoff + status detail

- Replace fixed 3s reconnect with exponential backoff
  (1s/2s/4s/8s/16s/30s, capped). Reduces console noise when
  upstream WebSocket is blocked (e.g. DSM reverse proxy without
  WS upgrade headers).
- ws.onerror swallowed (onclose still schedules reconnect) so the
  browser stops printing an unhandled-error pair per attempt.
- Expose reconnectAttempt in hook; TopBar shows 'Connecting…'
  pre-first-attempt and 'Disconnected · 재연결 시도 #N' after.

Root cause of WS failure is upstream (curl proves the endpoint
itself is fine — see DSM reverse proxy WebSocket headers).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/agent-office/hooks/useAgentManager.js

@@ -1,25 +1,34 @@
 // src/pages/agent-office/hooks/useAgentManager.js
 import { useState, useEffect, useRef, useCallback } from 'react';
 
-const WS_RECONNECT_DELAY = 3000;
+// Exponential backoff with cap. 1s → 2s → 4s → 8s → 16s → 30s (cap)
+const WS_BACKOFF_BASE_MS = 1000;
+const WS_BACKOFF_CAP_MS = 30000;
+const WS_BACKOFF_MAX_EXP = 5;
 
 export function useAgentManager() {
   const [agents, setAgents] = useState({});           // { agentId: { state, detail, task_id } }
   const [pendingTasks, setPendingTasks] = useState([]); // [{id, agent_id, task_type, input_data}]
   const [notifications, setNotifications] = useState({}); // { agentId: count }
   const [connected, setConnected] = useState(false);
+  const [reconnectAttempt, setReconnectAttempt] = useState(0);
   const [refreshTrigger, setRefreshTrigger] = useState(0); // 탭 데이터 리프레시용
 
   const wsRef = useRef(null);
   const reconnectRef = useRef(null);
   const connectRef = useRef(null);
+  const attemptRef = useRef(0);
 
   const connect = useCallback(() => {
     const protocol = window.location.protocol === 'https:' ? 'wss' : 'ws';
     const ws = new WebSocket(`${protocol}://${window.location.host}/api/agent-office/ws`);
     wsRef.current = ws;
 
-    ws.onopen = () => setConnected(true);
+    ws.onopen = () => {
+      setConnected(true);
+      attemptRef.current = 0;
+      setReconnectAttempt(0);
+    };
 
     ws.onmessage = (e) => {
       const msg = JSON.parse(e.data);
@@ -69,10 +78,17 @@ export function useAgentManager() {
 
     ws.onclose = () => {
       setConnected(false);
-      reconnectRef.current = setTimeout(() => connectRef.current?.(), WS_RECONNECT_DELAY);
+      const exp = Math.min(attemptRef.current, WS_BACKOFF_MAX_EXP);
+      const delay = Math.min(WS_BACKOFF_BASE_MS * 2 ** exp, WS_BACKOFF_CAP_MS);
+      attemptRef.current += 1;
+      setReconnectAttempt(attemptRef.current);
+      reconnectRef.current = setTimeout(() => connectRef.current?.(), delay);
     };
 
-    ws.onerror = () => ws.close();
+    // onerror fires before onclose; swallow so the browser doesn't print an
+    // unhandled-error pair for every retry. onclose still runs and schedules
+    // the next attempt.
+    ws.onerror = () => {};
   }, []);
 
   useEffect(() => {
@@ -108,6 +124,7 @@ export function useAgentManager() {
     pendingTasks,
     notifications,
     connected,
+    reconnectAttempt,
     refreshTrigger,
     sendCommand,
     sendApproval,