fix: SIP disconnect on callState change + dispose sends agentId

- Fixed useEffect dependency bug: callState in deps caused cleanup
  (disconnectSip) to fire on every state transition, killing SIP
  mid-dial. Now uses useRef for callState in beforeunload handler
  with empty deps array — cleanup only fires on unmount.
- sendBeacon auto-dispose now includes agentId from agent config
- Disposition modal submit now includes agentId from agent config

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

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

@@ -90,9 +90,11 @@ export const ActiveCallCard = ({ lead, callerPhone, missedCallId, onCallComplete
 
         // Submit disposition to sidecar
         if (callUcid) {
+            const agentCfg = JSON.parse(localStorage.getItem('helix_agent_config') ?? '{}');
             const disposePayload = {
                 ucid: callUcid,
                 disposition,
+                agentId: agentCfg.ozonetelAgentId,
                 callerPhone,
                 direction: callDirectionRef.current,
                 durationSec: callDuration,