feat(explorer): add image support, upload, and download to file explorer ...

- Backend: Added /fs/upload (multipart) and /fs/download (streaming) endpoints.\n- Backend: Fixed binary handling in fs_touch.\n- Frontend: Added nodeFsUpload and nodeFsDownloadBlob to apiService.\n- Frontend: Integrated image detection and preview lightbox.\n- Frontend: Added file upload action for folders and root.\n- Frontend: Added individual file download buttons.

refactor: re-org frontend folder structure to simply be 'frontend' ...

- Moved ui/client-app to frontend/

- Updated README and documentation references

- Updated docker-compose, local run scripts, and remote deploy scripts to point to new frontend location