fix(05-03): register alert callback to receive backend mixer changes

PROBLEM:
- onBackendMixerChanged never being called
- No [HandleAlertCallback] logs
- Backing track changes not triggering session refresh

ROOT CAUSE:
- Alert callback never registered with native client
- In legacy code: jamClient.SessionSetAlertCallback("JK.AlertCallback")
- React app had no equivalent setup
- Backend mixer change alerts (type=2) never delivered to app
- So onBackendMixerChanged never called, session never refreshed

SOLUTION:
1. Add JK.HandleAlertCallback global function in JKSessionScreen
2. Routes BACKEND_MIXER_CHANGE (type=2) to sessionModel.onBackendMixerChanged
3. Register callback with jamClient.SessionSetAlertCallback on session join
4. Unregister callback on session leave for cleanup

FLOW NOW:
1. User opens backing track via jamClient
2. Native client detects change
3. Native client calls JK.HandleAlertCallback(type=2, 'RebuildMediaControl')
4. Handler routes to sessionModel.onBackendMixerChanged()
5. Syncs tracks and calls refreshCurrentSession()
6. REST API returns session with participants[].backing_tracks
7. updateSessionInfo() extracts and dispatches to Redux
8. mixerHelper.backingTracks populated
9. Component displays backing track

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/components/client/JKSessionScreen.js

@@ -266,6 +266,33 @@ const JKSessionScreen = () => {
     }
   }, [sessionIdFromUrl, sessionId, dispatch]);
 
+  // Setup global alert callback handler for backend mixer changes
+  useEffect(() => {
+    if (!window.JK) window.JK = {};
+
+    // Alert callback handler - routes backend alerts to appropriate handlers
+    window.JK.HandleAlertCallback = (type, text) => {
+      console.log('[HandleAlertCallback] Alert received:', { type, text });
+
+      // BACKEND_MIXER_CHANGE = 2
+      if (type === 2) {
+        console.log('[HandleAlertCallback] BACKEND_MIXER_CHANGE detected:', text);
+        if (sessionModel && sessionModel.onBackendMixerChanged) {
+          sessionModel.onBackendMixerChanged(type, text);
+        } else {
+          console.warn('[HandleAlertCallback] sessionModel.onBackendMixerChanged not available');
+        }
+      }
+      // Handle other alert types as needed
+    };
+
+    return () => {
+      if (window.JK) {
+        delete window.JK.HandleAlertCallback;
+      }
+    };
+  }, [sessionModel]);
+
   useEffect(() => {
     if (!isConnected || !jamClient) return;
     console.debug("JKSessionScreen: -DEBUG- isConnected changed to true");
@@ -357,6 +384,7 @@ const JKSessionScreen = () => {
   const joinSession = async () => {
 
     await jamClient.SessionRegisterCallback("JK.HandleBridgeCallback2");
+    await jamClient.SessionSetAlertCallback("JK.HandleAlertCallback");
     await jamClient.RegisterRecordingCallbacks("JK.HandleRecordingStartResult", "JK.HandleRecordingStopResult", "JK.HandleRecordingStarted", "JK.HandleRecordingStopped", "JK.HandleRecordingAborted");
 
     await jamClient.SessionSetConnectionStatusRefreshRate(1000);

jam-ui/src/hooks/useSessionModel.js

@@ -493,6 +493,7 @@ export default function useSessionModel(app, server, sessionScreen) {
       // Unregister callbacks
       try {
         await jamClient.SessionRegisterCallback("");
+        await jamClient.SessionSetAlertCallback("");
         await jamClient.SessionSetConnectionStatusRefreshRate(0);
         logger.debug("Callbacks unregistered successfully");
       } catch (error) {