Add cache-control headers to front_api

[cristian-tamblay]

Summary

Improves how the frontend index.html file is served by adding cache-control headers.
This ensures that users always receive the latest version of the React build after a new release, avoiding the need to manually refresh with Shift + F5.

Type of change

• Back end new feature.

Changes

• Updated DashAI/back/api/front_api.py to add Cache-Control, Pragma, and Expires headers when serving index.html.
• Prevents outdated frontend builds from being loaded from browser cache.

How to Test

1. Build the frontend with yarn build.
2. Run the backend (python -m DashAI or the built .exe).
3. Open the app in your browser (http://localhost:8000/app/).
4. Deploy a new build and reload — the updated frontend should now load automatically without needing a hard refresh.

Notes

This change only affects the index.html route.
Static assets (JS, CSS, images) remain cached normally since their filenames are hash-based.