fix(26): pass jamServer to loadJamTrack and improve error handling

- Add jamServer to useMediaActions by creating object from subscribe/unsubscribe
- Pass jamServer to loadJamTrackThunk so it can subscribe to packaging progress
- Improve error handling to extract message from Response objects (not just Error)
- Fixes "Failed to start packaging: undefined" error

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

jam-ui/src/hooks/useMediaActions.js

@@ -1,4 +1,4 @@
-import { useCallback } from 'react';
+import { useCallback, useMemo } from 'react';
 import { useDispatch, useSelector } from 'react-redux';
 import {
   openBackingTrack as openBackingTrackThunk,
@@ -32,7 +32,13 @@ import { syncTracksToServer } from '../services/trackSyncService';
 const useMediaActions = () => {
   const dispatch = useDispatch();
   const sessionId = useSelector(selectSessionId);
-  const { jamClient } = useJamServerContext();
+  const { jamClient, subscribe, unsubscribe } = useJamServerContext();
+
+  // Create jamServer object with subscribe/unsubscribe for thunks
+  const jamServer = useMemo(() => ({
+    subscribe,
+    unsubscribe
+  }), [subscribe, unsubscribe]);
 
   /**
    * Open a backing track file
@@ -144,7 +150,7 @@ const useMediaActions = () => {
    */
   const loadJamTrack = useCallback(async (jamTrack) => {
     try {
-      await dispatch(loadJamTrackThunk({ jamTrack, jamClient })).unwrap();
+      await dispatch(loadJamTrackThunk({ jamTrack, jamClient, jamServer })).unwrap();
 
       // Update media summary
       dispatch(updateMediaSummary({
@@ -161,7 +167,7 @@ const useMediaActions = () => {
       console.error('Error loading jam track:', error);
       throw error;
     }
-  }, [dispatch, jamClient, sessionId]);
+  }, [dispatch, jamClient, jamServer, sessionId]);
 
   /**
    * Stop and close the currently playing JamTrack

jam-ui/src/store/features/mediaSlice.js

@@ -148,7 +148,9 @@ export const downloadJamTrack = createAsyncThunk(
         });
       } catch (err) {
         console.error('[JamTrack] Failed to enqueue mixdown:', err);
-        throw new Error(`Failed to start packaging: ${err.message}`);
+        // Handle both Error objects and Response objects (from apiFetch)
+        const errorMessage = err?.message || err?.statusText || `HTTP ${err?.status}` || 'Unknown error';
+        throw new Error(`Failed to start packaging: ${errorMessage}`);
       }
 
       // Subscribe to WebSocket notifications for packaging progress