feat: implement mixer categorization by group_id

Adds useEffect to categorize mixers into track-specific arrays:
- metronomeTrackMixers (group_id: 16)
- backingTrackMixers (group_id: 11)
- jamTrackMixers (group_id: 12)
- recordingTrackMixers (group_id: 13)
- adhocTrackMixers (other types)

Each mixer categorized as {master, personal, mixer} pair.
Dispatches to Redux for component consumption.

Resolves TODO at line 243 about groupMixersByType logic.

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

jam-ui/src/hooks/useMixerHelper.js

@@ -34,7 +34,12 @@ import {
   setMetronome as setMetronomeAction,
   setMediaSummary,
   setSimulatedMusicCategoryMixers as setSimulatedMusicAction,
-  setSimulatedChatCategoryMixers as setSimulatedChatAction
+  setSimulatedChatCategoryMixers as setSimulatedChatAction,
+  setRecordingTrackMixers,
+  setBackingTrackMixers,
+  setJamTrackMixers,
+  setMetronomeTrackMixers,
+  setAdhocTrackMixers
 } from '../store/features/mixersSlice';
 import {
   selectBackingTracks,
@@ -236,8 +241,68 @@ const useMixerHelper = () => {
     dispatch(organizeMixers());
   }, [currentSession, masterMixers, personalMixers, dispatch]);
 
-  // Note: groupMixersByType logic is handled by WebSocket MIXER_CHANGES handler
-  // which updates backingTrackMixers, jamTrackMixers, etc. via Redux actions
+  // Categorize mixers by group_id and dispatch to Redux
+  useEffect(() => {
+    if (!masterMixers || !personalMixers || masterMixers.length === 0) return;
+
+    console.log('[useMixerHelper] Categorizing mixers by group_id');
+
+    const metronomeTrackMixers = [];
+    const backingTrackMixers = [];
+    const jamTrackMixers = [];
+    const recordingTrackMixers = [];
+    const adhocTrackMixers = [];
+
+    // Process master/personal mixer pairs
+    for (const masterMixer of masterMixers) {
+      const personalMixer = personalMixers.find(p => p.rid === masterMixer.rid);
+      const mixerPair = {
+        master: masterMixer,
+        personal: personalMixer,
+        mixer: mixMode === MIX_MODES.PERSONAL ? personalMixer : masterMixer
+      };
+
+      // Categorize by group_id (from ChannelGroupIds)
+      switch (masterMixer.group_id) {
+        case ChannelGroupIds.MetronomeGroup: // 16
+          metronomeTrackMixers.push(mixerPair);
+          console.log('[useMixerHelper] Found metronome mixer:', masterMixer.id);
+          break;
+        case ChannelGroupIds.BackingTrackGroup: // 11
+          backingTrackMixers.push(mixerPair);
+          break;
+        case ChannelGroupIds.JamTrackGroup: // 12
+          jamTrackMixers.push(mixerPair);
+          break;
+        case ChannelGroupIds.RecordingGroup: // 13
+          recordingTrackMixers.push(mixerPair);
+          break;
+        default:
+          // Other mixer types go into adhoc
+          if (masterMixer.group_id !== ChannelGroupIds.UserMusicInputGroup &&
+              masterMixer.group_id !== ChannelGroupIds.ChatMicGroup &&
+              masterMixer.group_id !== ChannelGroupIds.BroadcastGroup) {
+            adhocTrackMixers.push(mixerPair);
+          }
+      }
+    }
+
+    console.log('[useMixerHelper] Categorized mixers:', {
+      metronome: metronomeTrackMixers.length,
+      backing: backingTrackMixers.length,
+      jam: jamTrackMixers.length,
+      recording: recordingTrackMixers.length,
+      adhoc: adhocTrackMixers.length
+    });
+
+    // Dispatch to Redux
+    dispatch(setMetronomeTrackMixers(metronomeTrackMixers));
+    dispatch(setBackingTrackMixers(backingTrackMixers));
+    dispatch(setJamTrackMixers(jamTrackMixers));
+    dispatch(setRecordingTrackMixers(recordingTrackMixers));
+    dispatch(setAdhocTrackMixers(adhocTrackMixers));
+
+  }, [masterMixers, personalMixers, mixMode, dispatch]);
 
   // Sync local isReady ref with Redux isReady state
   // This ensures VU meter callbacks have immediate access to the ready state