feat(05-04): implement download/sync UI with 6-state machine visualization

- Add handleCancelDownload callback to cancel downloads via JamTrackCancelDownload
- Add handleRetryDownload callback to retry failed downloads
- Implement 6-state sync machine UI (checking, downloading, keying, error)
- Show progress bar with percentage (0-100%) during download
- Show step indicator (X of Y) when available
- Display Cancel button during download state
- Display Retry button on error state with error message
- Hide UI for idle and synchronized states (normal playback)
- Import setDownloadState from mediaSlice

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

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

@@ -4,7 +4,8 @@ import {
   loadJamTrack,
   checkJamTrackSync,
   closeJamTrack,
-  setJamTrackState
+  setJamTrackState,
+  setDownloadState
 } from '../../store/features/mediaSlice';
 import { setOpenJamTrack, clearOpenJamTrack } from '../../store/features/sessionUISlice';
 import { setAvailableMixdowns, setActiveMixdown } from '../../store/features/activeSessionSlice';
@@ -281,6 +282,48 @@ const JKSessionJamTrackPlayer = ({
     }
   }, [isOperating, jamClient, jamTrack, jamTrackState, availableMixdowns, dispatch]);
 
+  // Download cancel handler
+  const handleCancelDownload = useCallback(async () => {
+    if (!jamClient || !fqIdRef.current) return;
+
+    try {
+      await jamClient.JamTrackCancelDownload(fqIdRef.current);
+      dispatch(setDownloadState({ state: 'idle', progress: 0 }));
+    } catch (err) {
+      console.error('[JamTrack] Cancel download error:', err);
+      setError({ type: 'download', message: 'Failed to cancel download' });
+    }
+  }, [jamClient, dispatch]);
+
+  // Download retry handler
+  const handleRetryDownload = useCallback(async () => {
+    if (isOperating || !jamClient) return;
+
+    try {
+      setIsOperating(true);
+      setError(null);
+
+      // Clear error state
+      dispatch(setDownloadState({ state: 'idle', error: null }));
+
+      // Retry load (will trigger download if not synced)
+      await dispatch(loadJamTrack({
+        jamTrack,
+        mixdownId: selectedMixdownId,
+        autoPlay: false,
+        jamClient
+      })).unwrap();
+
+    } catch (err) {
+      console.error('[JamTrack] Retry download error:', err);
+      setError({ type: 'download', message: 'Retry failed' });
+    } finally {
+      if (mountedRef.current) {
+        setIsOperating(false);
+      }
+    }
+  }, [isOperating, jamClient, jamTrack, selectedMixdownId, dispatch]);
+
   // Helper: Format milliseconds to MM:SS
   const formatTime = (ms) => {
     if (!ms || isNaN(ms)) return '00:00';
@@ -359,9 +402,43 @@ const JKSessionJamTrackPlayer = ({
         </div>
       )}
 
+{/* Download/Sync State Machine UI */}
       {downloadState.state !== 'idle' && downloadState.state !== 'synchronized' && (
-        <div>
-          <p>Download: {downloadState.state} ({downloadState.progress}%)</p>
+        <div style={{ background: '#f0f8ff', padding: '15px', marginBottom: '10px', border: '1px solid #cce7ff' }}>
+          <h4>
+            {downloadState.state === 'checking' && 'Checking sync status...'}
+            {downloadState.state === 'downloading' && 'Downloading JamTrack...'}
+            {downloadState.state === 'keying' && 'Requesting decryption keys...'}
+            {downloadState.state === 'error' && 'Download Failed'}
+          </h4>
+
+          {downloadState.state === 'downloading' && (
+            <div>
+              <progress
+                value={downloadState.progress}
+                max="100"
+                style={{ width: '100%', height: '20px' }}
+              />
+              <p>{downloadState.progress}%</p>
+              {downloadState.totalSteps > 0 && (
+                <p>Step {downloadState.currentStep} of {downloadState.totalSteps}</p>
+              )}
+              <button onClick={handleCancelDownload}>Cancel</button>
+            </div>
+          )}
+
+          {downloadState.state === 'keying' && (
+            <div>
+              <p>Finalizing download...</p>
+            </div>
+          )}
+
+          {downloadState.state === 'error' && (
+            <div style={{ color: '#d00' }}>
+              <p>{downloadState.error?.message || 'Download failed'}</p>
+              <button onClick={handleRetryDownload}>Retry</button>
+            </div>
+          )}
         </div>
       )}