fix(05-03): expose backingTracks, jamTracks, recordedTracks from useMixerHelper

Root cause fix: useMixerHelper was reading backingTracks from Redux
but not returning it in the hook's return object, causing
mixerHelper.backingTracks to always be undefined.

The issue:
- Line 88: Reads backingTracks from Redux mediaSlice
- Line 89: Reads jamTracks from Redux mediaSlice
- Line 90: Reads recordedTracks from Redux mediaSlice
- Line 831-847: Return object MISSING these three properties

This caused:
- WebSocket MIXER_CHANGES dispatches setBackingTracks() ✓
- Redux mediaSlice.backingTracks updates ✓
- useMixerHelper reads it via selector ✓
- But doesn't expose it to components ✗
- Result: mixerHelper.backingTracks always undefined

Fix:
Added backingTracks, jamTracks, recordedTracks to return object.

Now the full data flow works:
1. WebSocket MIXER_CHANGES arrives
2. Dispatches setBackingTracks(mixers.backingTracks)
3. Redux mediaSlice updates
4. useMixerHelper reads via selector
5. Returns in hook object
6. Components access via mixerHelper.backingTracks

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

jam-ui/src/hooks/useMixerHelper.js

@@ -843,7 +843,11 @@ const useMixerHelper = () => {
     mixMode,
     faderChanged,
     initGain,
-    setMixerPan
+    setMixerPan,
+    // Media arrays from mediaSlice
+    backingTracks,
+    jamTracks,
+    recordedTracks
   };
 }