feat(03-02): add loading states and disabled UI

Implemented loading states and disabled controls during operations:

Loading states:
- isLoadingDuration: Tracks duration fetch on track open
- isOperating: Prevents rapid clicks on play/stop buttons

Duration loading:
- Shows "Loading track..." indicator while fetching
- Disables all controls until ready
- Sets/clears loading state in fetchDuration

Operation flags:
- handlePlay: Checks isOperating, uses finally block to reset
- handleStop: Checks isOperating, uses finally block to reset
- handleSeek: Allows immediate UI update (no blocking)

Button disabled logic:
- Play/Pause: disabled when (!backingTrack || isLoadingDuration || isOperating || error)
- Stop: disabled when (!backingTrack || isLoadingDuration || isOperating)
- Slider: disabled when (!backingTrack || isLoadingDuration)

Loading indicators:
- "Loading track..." text during duration fetch
- "..." suffix on Play/Pause button during operations
- Applied to both popup and modal versions

Prevents rapid-click issues with isOperating guard.

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

@@ -30,6 +30,10 @@ const JKSessionBackingTrackPlayer = ({
   const [error, setError] = useState(null);
   const [errorType, setErrorType] = useState(null); // 'file', 'network', 'playback', 'general'
 
+  // Loading states
+  const [isLoadingDuration, setIsLoadingDuration] = useState(true);
+  const [isOperating, setIsOperating] = useState(false); // Operation in progress
+
   const volumeRef = useRef(null);
   const trackVolumeObjectRef = useRef({
     volL: 0,
@@ -84,9 +88,11 @@ const JKSessionBackingTrackPlayer = ({
 
       // Fetch and set duration immediately when track loads (async)
       const fetchDuration = async () => {
+        setIsLoadingDuration(true);
         try {
           if (!jamClient) {
             handleError('general', 'Audio engine not available', null);
+            setIsLoadingDuration(false);
             return;
           }
 
@@ -101,16 +107,19 @@ const JKSessionBackingTrackPlayer = ({
             handleError('file', 'Failed to load track duration. The file may be invalid or unsupported.', null);
             setDuration('0:00');
             setDurationMs(0);
+            setIsLoadingDuration(false);
             return;
           }
 
           setDurationMs(validDuration);
           setDuration(formatTime(validDuration));
           clearError(); // Clear any previous errors
+          setIsLoadingDuration(false);
         } catch (error) {
           handleError('file', 'Failed to load track duration', error);
           setDuration('0:00');
           setDurationMs(0);
+          setIsLoadingDuration(false);
         }
       };
 
@@ -183,6 +192,9 @@ const JKSessionBackingTrackPlayer = ({
   }, [isPlaying, jamClient, backingTrack]);
 
   const handlePlay = async () => {
+    if (isOperating) return; // Prevent rapid clicks
+    setIsOperating(true);
+
     try {
       if (!jamClient) {
         handleError('general', 'Audio engine not available', null);
@@ -248,10 +260,15 @@ const JKSessionBackingTrackPlayer = ({
       handleError('playback', 'Failed to start playback', error);
       // Reset isPlaying to ensure UI is consistent
       setIsPlaying(false);
+    } finally {
+      setIsOperating(false);
     }
   };
 
   const handleStop = async () => {
+    if (isOperating) return; // Prevent rapid clicks
+    setIsOperating(true);
+
     try {
       if (!jamClient) {
         handleError('general', 'Audio engine not available', null);
@@ -273,6 +290,8 @@ const JKSessionBackingTrackPlayer = ({
       setCurrentTime('0:00');
       setCurrentPositionMs(0);
       pendingSeekPositionRef.current = null;
+    } finally {
+      setIsOperating(false);
     }
   };
 
@@ -371,6 +390,10 @@ const JKSessionBackingTrackPlayer = ({
     const seekPositionMs = parseInt(e.target.value);
     const previousPosition = currentPositionMs;
 
+    // Update local state immediately for responsive UI (don't block on isOperating)
+    setCurrentPositionMs(seekPositionMs);
+    setCurrentTime(formatTime(seekPositionMs));
+
     try {
       if (!jamClient) {
         handleError('general', 'Audio engine not available', null);
@@ -379,10 +402,6 @@ const JKSessionBackingTrackPlayer = ({
 
       console.log('[BTP] handleSeek:', { seekPositionMs, isPlaying });
 
-      // Update local state immediately for responsive UI
-      setCurrentPositionMs(seekPositionMs);
-      setCurrentTime(formatTime(seekPositionMs));
-
       // Seek the native client to the new position
       await jamClient.SessionTrackSeekMs(seekPositionMs);
 
@@ -516,21 +535,29 @@ const JKSessionBackingTrackPlayer = ({
                 </div>
               )}
 
+              {/* Loading indicator */}
+              {isLoadingDuration && (
+                <div style={{ textAlign: 'center', color: '#6c757d', fontSize: '14px' }}>
+                  Loading track...
+                </div>
+              )}
+
               {/* Row 2: Controls */}
               <div style={{ display: 'flex', flexDirection: 'column', alignItems: 'center', gap: '8px' }}>
                 <div style={{ display: 'flex', justifyContent: 'center', alignItems: 'center', gap: '8px' }}>
                   <Button
                     color="primary"
                     onClick={handlePlay}
-                    disabled={!backingTrack}
+                    disabled={!backingTrack || isLoadingDuration || isOperating || error}
                   >
                     <FontAwesomeIcon icon={isPlaying ? faPause : faPlay} />
+                    {isOperating && ' ...'}
                   </Button>
 
                   <Button
                     color="secondary"
                     onClick={handleStop}
-                    disabled={!backingTrack}
+                    disabled={!backingTrack || isLoadingDuration || isOperating}
                   >
                     <FontAwesomeIcon icon={faStop} />
                   </Button>
@@ -546,7 +573,7 @@ const JKSessionBackingTrackPlayer = ({
                     value={currentPositionMs}
                     onChange={handleSeek}
                     style={{ flex: 1 }}
-                    disabled={!backingTrack}
+                    disabled={!backingTrack || isLoadingDuration}
                   />
                   <span style={{ fontSize: '14px', minWidth: '40px' }}>{duration}</span>
                 </div>
@@ -637,21 +664,29 @@ const JKSessionBackingTrackPlayer = ({
             </div>
           )}
 
+          {/* Loading indicator */}
+          {isLoadingDuration && (
+            <div style={{ textAlign: 'center', color: '#6c757d', fontSize: '14px' }}>
+              Loading track...
+            </div>
+          )}
+
           {/* Row 2: Controls */}
           <div style={{ display: 'flex', flexDirection: 'column', alignItems: 'center', gap: '8px' }}>
             <div style={{ display: 'flex', justifyContent: 'center', alignItems: 'center', gap: '8px' }}>
               <Button
                 color="primary"
                 onClick={handlePlay}
-                disabled={!backingTrack}
+                disabled={!backingTrack || isLoadingDuration || isOperating || error}
               >
                 <FontAwesomeIcon icon={isPlaying ? faPause : faPlay} /> Play/Pause
+                {isOperating && ' ...'}
               </Button>
 
               <Button
                 color="secondary"
                 onClick={handleStop}
-                disabled={!backingTrack}
+                disabled={!backingTrack || isLoadingDuration || isOperating}
               >
                 <FontAwesomeIcon icon={faStop} /> Stop
               </Button>
@@ -667,7 +702,7 @@ const JKSessionBackingTrackPlayer = ({
                 value={currentPositionMs}
                 onChange={handleSeek}
                 style={{ flex: 1 }}
-                disabled={!backingTrack}
+                disabled={!backingTrack || isLoadingDuration}
               />
               <span style={{ fontSize: '14px', minWidth: '40px' }}>{duration}</span>
             </div>