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/AgentOffice.jsx

@@ -10,7 +10,7 @@ import './AgentOffice.css';
 
 export default function AgentOffice() {
   const {
-    agents, pendingTasks, notifications, connected,
+    agents, pendingTasks, notifications, connected, reconnectAttempt,
     refreshTrigger, clearNotifications
   } = useAgentManager();
 
@@ -53,7 +53,7 @@ export default function AgentOffice() {
 
   return (
     <div className="ao-root">
-      <TopBar connected={connected} />
+      <TopBar connected={connected} reconnectAttempt={reconnectAttempt} />
       <div className="ao-main">
         <div className="ao-grid-wrap">
           <AgentGrid