feat(frontend): supervisor presence indicator on agent call card

- useAgentState hook returns { state, supervisorPresence }
- SSE events: supervisor-whisper → "Supervisor coaching" (blue badge)
  supervisor-barge → "Supervisor on call" (brand badge)
  supervisor-left → badge disappears
- Listen mode is silent — no badge shown
- Updated call sites: sidebar.tsx, agent-status-toggle.tsx destructure

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/use-agent-state.ts

@@ -2,11 +2,13 @@ import { useState, useEffect, useRef } from 'react';
 import { notify } from '@/lib/toast';
 
 export type OzonetelState = 'ready' | 'break' | 'training' | 'calling' | 'in-call' | 'acw' | 'offline';
+export type SupervisorPresence = 'none' | 'whisper' | 'barge';
 
 const API_URL = import.meta.env.VITE_API_URL ?? 'http://localhost:4100';
 
-export const useAgentState = (agentId: string | null): OzonetelState => {
+export const useAgentState = (agentId: string | null): { state: OzonetelState; supervisorPresence: SupervisorPresence } => {
     const [state, setState] = useState<OzonetelState>('offline');
+    const [supervisorPresence, setSupervisorPresence] = useState<SupervisorPresence>('none');
     const prevStateRef = useRef<OzonetelState>('offline');
     const esRef = useRef<EventSource | null>(null);
 
@@ -56,6 +58,20 @@ export const useAgentState = (agentId: string | null): OzonetelState => {
                     return;
                 }
 
+                // Supervisor presence events — don't replace agent state
+                if (data.state === 'supervisor-whisper') {
+                    setSupervisorPresence('whisper');
+                    return;
+                }
+                if (data.state === 'supervisor-barge') {
+                    setSupervisorPresence('barge');
+                    return;
+                }
+                if (data.state === 'supervisor-left') {
+                    setSupervisorPresence('none');
+                    return;
+                }
+
                 prevStateRef.current = data.state;
                 setState(data.state);
             } catch {
@@ -74,5 +90,5 @@ export const useAgentState = (agentId: string | null): OzonetelState => {
         };
     }, [agentId]);
 
-    return state;
+    return { state, supervisorPresence };
 };