fix(27): correct MediaTrackGroup ID from 8 to 6

MediaTrackGroup is 6 (not 8) per globals.js ChannelGroupIds.
8 is StreamOutChatGroup, which is why no backing tracks were found.

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

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

@@ -1,8 +1,8 @@
 import { createSlice, createAsyncThunk } from '@reduxjs/toolkit';
 import { enqueueMixdown } from '../../helpers/rest';
 
-// Channel group IDs for track filtering
-const MEDIA_TRACK_GROUP = 8; // ChannelGroupIds.MediaTrackGroup
+// Channel group IDs for track filtering (from globals.js ChannelGroupIds)
+const MEDIA_TRACK_GROUP = 6; // ChannelGroupIds.MediaTrackGroup
 
 // Async thunks for media actions
 export const openBackingTrack = createAsyncThunk(