feat: dockerized the whole , single command run available now

[Satvik-Singh192]

Description

Added full Docker support for the MERN Mass Email Sender project.
The entire stack (frontend, backend, and MongoDB) can now be built and run using a single command with Docker Compose --build.
This simplifies setup, ensures consistency across environments, and eliminates the need for manual installation of Node.js or MongoDB.

Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

Issues

Closes #55

Screenshots

--- --- ---

Checklist

• I have read the Contributing Guidelines.
• Added Dockerfiles for backend and frontend
• Added docker-compose.yml for full stack setup
• Added .dockerignore files
• Updated README with Docker usage instructions

[Satvik-Singh192]

@MD-IRFAN-RAJ please review my pull request sir