feat(05-04): fetch and organize mixdowns in player initialization

- Add JamTrackGetMixdowns API call during player initialization
- Organize mixdowns into master/customMixes/stems hierarchy
- Store availableMixdowns in Redux (activeSessionSlice)
- Set default mixdown selection (prefer master, fallback to first)
- Set activeMixdown in Redux for UI display
- Non-fatal error handling (can play without explicit selection)

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

jam-ui/src/components/client/JKSessionJamTrackPlayer.js

@@ -7,6 +7,7 @@ import {
   setJamTrackState
 } from '../../store/features/mediaSlice';
 import { setOpenJamTrack, clearOpenJamTrack } from '../../store/features/sessionUISlice';
+import { setAvailableMixdowns, setActiveMixdown } from '../../store/features/activeSessionSlice';
 
 const JKSessionJamTrackPlayer = ({
   isOpen,
@@ -65,6 +66,40 @@ const JKSessionJamTrackPlayer = ({
           console.log('[JamTrack] Not synchronized, will download');
         }
 
+        // Fetch available mixdowns
+        try {
+          const mixdowns = await jamClient.JamTrackGetMixdowns(jamTrack.id);
+
+          if (mixdowns && mixdowns.length > 0) {
+            // Organize into hierarchy: master, custom mixes, stems
+            const organized = {
+              master: mixdowns.find(m => m.type === 'master') || null,
+              customMixes: mixdowns.filter(m => m.type === 'custom' || m.type === 'custom-mix'),
+              stems: mixdowns.filter(m => m.type === 'stem')
+            };
+
+            dispatch(setAvailableMixdowns(mixdowns));
+
+            // Set initial mixdown if not already selected
+            if (!selectedMixdownId) {
+              const defaultMixdown = organized.master || (mixdowns[0] || null);
+              if (defaultMixdown) {
+                setSelectedMixdownId(defaultMixdown.id);
+                dispatch(setActiveMixdown(defaultMixdown));
+              }
+            } else {
+              // Find and set the initially selected mixdown
+              const mixdown = mixdowns.find(m => m.id === selectedMixdownId);
+              if (mixdown) {
+                dispatch(setActiveMixdown(mixdown));
+              }
+            }
+          }
+        } catch (err) {
+          console.error('[JamTrack] Failed to fetch mixdowns:', err);
+          // Non-fatal - can still play default mixdown
+        }
+
         // Set open in Redux
         dispatch(setOpenJamTrack(jamTrack.id));