Improve error reporting for users

## Summary
Currently, when errors occur in the FReD Explorer or FLoRA Annotator apps, users don't have a clear way to understand what went wrong or how to report the issue.

## Suggested improvements

1. **User-friendly error messages**: Replace technical error messages with clear, actionable descriptions
2. **Error toast/notification system**: Show non-blocking error notifications that don't disrupt the user flow
3. **Include context in error reports**: When an error occurs, provide an easy way to include relevant context (e.g., what action was being performed, browser info, data state)
4. **Pre-fill error reports**: Consider adding a "Report this error" button that opens the feedback email with error details pre-filled
5. **Console logging for debugging**: Ensure all errors are logged to console with full stack traces for developers

## Areas to review
- Data loading errors (CSV parsing, network failures)
- DOI lookup failures
- PDF parsing errors
- Chart rendering errors
- DataTables initialization/update errors

## Related
A feedback link has been added to the footer of both apps to make it easier for users to report issues.