feat(16-01): add upload success toast notification

- Add selectUploadStatus selector to track upload state transitions
- Track previous upload status with useRef
- Show toast.success when upload completes (uploading -> idle)
- Auto-dismiss after 3 seconds per REQ-5.4
- Success toast provides clear feedback on successful file uploads

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

@@ -55,7 +55,7 @@ import {
   selectBackingTrackData,
   selectJamTrackData
 } from '../../store/features/activeSessionSlice';
-import { addMessageFromWebSocket, uploadAttachment, selectIsUploading, selectUploadError, selectUploadFileName, clearUploadError } from '../../store/features/sessionChatSlice';
+import { addMessageFromWebSocket, uploadAttachment, selectIsUploading, selectUploadError, selectUploadFileName, selectUploadStatus, clearUploadError } from '../../store/features/sessionChatSlice';
 import { validateFile } from '../../services/attachmentValidation';
 
 import { CLIENT_ROLE, RECORD_TYPE_AUDIO, RECORD_TYPE_BOTH } from '../../helpers/globals';
@@ -203,6 +203,8 @@ const JKSessionScreen = () => {
   const isUploading = useSelector(selectIsUploading);
   const uploadError = useSelector(selectUploadError);
   const uploadFileName = useSelector(selectUploadFileName);
+  const uploadStatus = useSelector(selectUploadStatus);
+  const prevUploadStatusRef = useRef(uploadStatus);
 
   // File input ref for attach button
   const attachFileInputRef = useRef(null);
@@ -1048,6 +1050,15 @@ const JKSessionScreen = () => {
     }
   }, [uploadError, dispatch]);
 
+  // Show success toast when upload completes
+  useEffect(() => {
+    // Show success toast when upload transitions from 'uploading' to 'idle'
+    if (prevUploadStatusRef.current === 'uploading' && uploadStatus === 'idle') {
+      toast.success('File uploaded successfully', { autoClose: 3000 });
+    }
+    prevUploadStatusRef.current = uploadStatus;
+  }, [uploadStatus]);
+
   const handleBackingTrackSelected = async (result) => {
     console.log('JKSessionScreen: handleBackingTrackSelected called with:', result);
     console.log('JKSessionScreen: Current state - showBackingTrackPopup:', showBackingTrackPopup, 'popupGuard:', popupGuard);