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/components/call-desk/active-call-card.tsx

@@ -56,6 +56,18 @@ export const ActiveCallCard = ({ lead, callerPhone, missedCallId, onCallComplete
         console.log(`[ACTIVE-CALL-CARD] Mounted: state=${callState} direction=${callDirectionRef.current} ucid=${callUcid ?? 'none'} lead=${lead?.id ?? 'none'} phone=${callerPhone}`);
     }, []); // eslint-disable-line react-hooks/exhaustive-deps
 
+    // Sync UCID to localStorage so sendBeacon can fire auto-dispose on page refresh.
+    // Cleared on disposition submit (handleDisposition below) or when call resets to idle.
+    useEffect(() => {
+        if (callUcid) {
+            localStorage.setItem('helix_active_ucid', callUcid);
+        }
+        return () => {
+            // Don't clear on unmount if disposition hasn't fired — the
+            // beforeunload handler in SipProvider needs it
+        };
+    }, [callUcid]);
+
     // Detect caller disconnect: call was active and ended without agent pressing End
     useEffect(() => {
         if (wasAnsweredRef.current && !dispositionOpen && (callState === 'ended' || callState === 'failed')) {
@@ -115,6 +127,9 @@ export const ActiveCallCard = ({ lead, callerPhone, missedCallId, onCallComplete
             }
         }
 
+        // Clear persisted UCID — disposition was submitted, no need for sendBeacon fallback
+        localStorage.removeItem('helix_active_ucid');
+
         notify.success('Call Logged', `Disposition: ${disposition.replace(/_/g, ' ').toLowerCase()}`);
         handleReset();
     };