fix(16-01): add S3 404 error handling with toast notification

- Add react-toastify import for toast notifications
- Show toast.error('File no longer available') when attachment fetch fails
- Handles S3 404 errors gracefully per REQ-5.5
- Replaces silent failure with user-friendly error message

jam-ui/src/components/client/chat/JKChatMessage.js

@@ -1,5 +1,6 @@
 import React, { useCallback, useState } from 'react';
 import PropTypes from 'prop-types';
+import { toast } from 'react-toastify';
 import { formatTimestamp } from '../../../utils/formatTimestamp';
 import { formatFileSize } from '../../../services/attachmentValidation';
 import { getMusicNotationUrl } from '../../../helpers/rest';
@@ -63,7 +64,8 @@ const JKChatMessage = ({ message }) => {
         }
       } catch (error) {
         console.error('Failed to fetch attachment URL:', error);
-        // Could show toast error here, but keep it simple for now
+        // Show user-friendly error (REQ-5.5)
+        toast.error('File no longer available');
       } finally {
         setIsLoadingUrl(false);
       }