fix(05-03): extract backing tracks from session response to Redux

PROBLEM:
- mixerHelper.backingTracks was always empty/null
- Backing track not appearing in session screen
- useSessionWebSocket had MIXER_CHANGES callback that never fired

ROOT CAUSE:
- MIXER_CHANGES WebSocket message doesn't exist on server
- Backing track data comes from REST API /api/sessions/:id response
- Session data stored in activeSessionSlice but never extracted to mediaSlice
- useMixerHelper reads from mediaSlice.backingTracks which was never populated

SOLUTION:
- Import mediaSlice actions (setBackingTracks, setJamTracks, setRecordedTracks) in useSessionModel
- Update updateSessionInfo() to extract backing tracks from session.participants[].backing_tracks
- Dispatch extracted arrays to Redux mediaSlice when session updates
- Also extract jamTracks and recordedTracks for consistency

DATA FLOW:
1. User opens backing track via jamClient.SessionOpenBackingTrackFile()
2. Server updates session state
3. Server sends TRACKS_CHANGED WebSocket message
4. Client calls refreshCurrentSession() -> REST API /api/sessions/:id
5. Response includes participants with backing_tracks (containing mixers)
6. updateSessionInfo() extracts backing tracks and dispatches to Redux
7. useMixerHelper reads from mediaSlice.backingTracks
8. Components access via mixerHelper.backingTracks

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

jam-ui/src/hooks/useSessionModel.js

@@ -10,6 +10,11 @@ import {
   setSessionId,
   updateSessionData
 } from '../store/features/activeSessionSlice';
+import {
+  setBackingTracks,
+  setJamTracks,
+  setRecordedTracks
+} from '../store/features/mediaSlice';
 import useGearUtils from './useGearUtils';
 import useTrackHelpers from './useTrackHelpers';
 import useRecordingHelpers from './useRecordingHelpers';
@@ -675,13 +680,41 @@ export default function useSessionModel(app, server, sessionScreen) {
       // sendClientParticipantChanges logic would go here
 
       updateCurrentSession(response);
+
+      // Extract and dispatch media arrays to Redux
+      // Extract backing tracks from participants
+      let extractedBackingTracks = [];
+      if (response.participants) {
+        response.participants.forEach((participant) => {
+          if (participant.backing_tracks && participant.backing_tracks.length > 0) {
+            extractedBackingTracks = participant.backing_tracks;
+          }
+        });
+      }
+      dispatch(setBackingTracks(extractedBackingTracks));
+      console.log('[useSessionModel] Extracted backing tracks:', extractedBackingTracks);
+
+      // Extract jam tracks
+      const extractedJamTracks = response.jam_track && response.jam_track.tracks
+        ? response.jam_track.tracks.filter(track => track.track_type === 'Track')
+        : [];
+      dispatch(setJamTracks(extractedJamTracks));
+      console.log('[useSessionModel] Extracted jam tracks:', extractedJamTracks);
+
+      // Extract recorded tracks
+      const extractedRecordedTracks = response.claimed_recording && response.claimed_recording.recording
+        ? response.claimed_recording.recording.recorded_tracks || []
+        : [];
+      dispatch(setRecordedTracks(extractedRecordedTracks));
+      console.log('[useSessionModel] Extracted recorded tracks:', extractedRecordedTracks);
+
       if (callback) {
         callback();
       }
     } else {
       logger.info("ignoring refresh because we already have current: " + currentTrackChanges + ", seen: " + response.track_changes_counter);
     }
-  }, [currentTrackChanges, updateCurrentSession]);
+  }, [currentTrackChanges, updateCurrentSession, dispatch]);
 
   // Refresh current session REST call
   const refreshCurrentSessionRest = useCallback(async (callback, force) => {