feat(ui): convert clear chat confirm alert to custom modal ...

- Removed window.confirm for clearing chat history.\n- Added a modern React modal to prevent page flashing and improve UX.

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