fix: three-layer ACW protection — prevent agent stuck in wrapping-up

Root cause: when an agent refreshes the page during or after a call,
the React state (UCID, callState, disposition modal) is wiped. The
SIP BYE event fires but no component exists to trigger the disposition
modal → no POST to /api/ozonetel/dispose → agent stuck in ACW.

Layer 1 (beforeunload warning):
  Shows browser's native "Leave page?" dialog during active calls.
  Agent can cancel and stay.

Layer 2 (sendBeacon auto-dispose):
  UCID persisted to localStorage when call activates. On page unload,
  navigator.sendBeacon fires /api/ozonetel/dispose with
  CALLBACK_REQUESTED. Guaranteed delivery even during page death.
  Cleared from localStorage when disposition modal submits normally.

Layer 3 lives in helix-engage-server (separate commit).

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

src/providers/sip-provider.tsx

@@ -89,15 +89,43 @@ export const SipProvider = ({ children }: PropsWithChildren) => {
     // No auto-reset — the ActiveCallCard handles post-call flow (disposition → appointment → done)
     // and resets to idle via the "Back to Worklist" button
 
-    // Cleanup on unmount + page unload
+    // Layer 1: Warn on page refresh/close during active call
+    // Layer 2: Fire sendBeacon to auto-dispose if user confirms leave
+    // These two layers protect against the "agent refreshes mid-call → stuck in ACW" bug.
+    // Layer 3 (server-side ACW timeout) lives in supervisor.service.ts.
     useEffect(() => {
+        const handleBeforeUnload = (e: BeforeUnloadEvent) => {
+            const ucid = localStorage.getItem('helix_active_ucid');
+
+            // Layer 1: Show browser "Leave page?" dialog during active calls
+            if (callState === 'active' || callState === 'ringing-in' || callState === 'ringing-out') {
+                e.preventDefault();
+                e.returnValue = '';
+            }
+
+            // Layer 2: Fire disposition beacon if there's an active UCID
+            // sendBeacon is guaranteed to fire even during page unload
+            if (ucid) {
+                const payload = JSON.stringify({
+                    ucid,
+                    disposition: 'CALLBACK_REQUESTED',
+                    autoDisposed: true,
+                });
+                navigator.sendBeacon('/api/ozonetel/dispose', new Blob([payload], { type: 'application/json' }));
+                localStorage.removeItem('helix_active_ucid');
+            }
+        };
+
         const handleUnload = () => disconnectSip();
-        window.addEventListener('beforeunload', handleUnload);
+
+        window.addEventListener('beforeunload', handleBeforeUnload);
+        window.addEventListener('unload', handleUnload);
         return () => {
-            window.removeEventListener('beforeunload', handleUnload);
+            window.removeEventListener('beforeunload', handleBeforeUnload);
+            window.removeEventListener('unload', handleUnload);
             disconnectSip();
         };
-    }, []);
+    }, [callState]);
 
     return <>{children}</>;
 };