feat(05-02): add 3 WebSocket handlers for JamTrack events

- Extended MIXER_CHANGES: handles jamTrackMixdowns updates
- Enhanced JAM_TRACK_CHANGES: parses state correctly with parseInt
- Added MIXDOWN_CHANGES: tracks download progress steps
- All handlers dispatch correct Redux actions
- Added imports: setAvailableMixdowns, setDownloadState

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

jam-ui/src/hooks/useSessionWebSocket.js

@@ -15,7 +15,8 @@ import {
   startRecording,
   stopRecording,
   addRecordedTrack,
-  setConnectionStatus
+  setConnectionStatus,
+  setAvailableMixdowns
 } from '../store/features/activeSessionSlice';
 import {
   updateMediaSummary,
@@ -25,7 +26,8 @@ import {
   setBackingTracks,
   setJamTracks,
   setRecordedTracks,
-  updateJamTrackState
+  updateJamTrackState,
+  setDownloadState
 } from '../store/features/mediaSlice';
 
 /**
@@ -132,11 +134,31 @@ export const useSessionWebSocket = (sessionId) => {
         dispatch(setJamTracks(mixers.jamTracks || []));
         dispatch(setRecordedTracks(mixers.recordedTracks || []));
         dispatch(setMetronome(mixers.metronome || null));
+
+        // Phase 5 Plan 2: Handle JamTrack mixdown changes
+        if (mixers.jamTrackMixdowns) {
+          dispatch(setAvailableMixdowns(mixers.jamTrackMixdowns));
+        }
       },
 
       JAM_TRACK_CHANGES: (changes) => {
         console.log('Jam track changes received:', changes);
-        dispatch(updateJamTrackState(changes));
+        dispatch(updateJamTrackState({
+          isPlaying: changes.jamTrackState?.isPlaying || false,
+          isPaused: changes.jamTrackState?.isPaused || false,
+          currentPositionMs: parseInt(changes.jamTrackState?.currentPositionMs || '0', 10),
+          lastUpdate: Date.now()
+        }));
+      },
+
+      MIXDOWN_CHANGES: (message) => {
+        console.log('Mixdown changes received:', message);
+        if (message.mixdownPackage) {
+          dispatch(setDownloadState({
+            currentStep: message.mixdownPackage.current_packaging_step || 0,
+            totalSteps: message.mixdownPackage.packaging_steps || 0
+          }));
+        }
       },
 
       // Connection events