Refactor (monorepo): refactor backend for monorepo setup

[ChayanDass]

Changes

• Moved frontend code into the backend directory → created a monorepo layout.
• Added Dockerfile for frontend and updated docker-compose.yml to run backend + frontend together.
• Added Makefile with commands for:
  • Linting & formatting (Go + frontend).
  • Running frontend/backend locally.
  • Cleaning test caches and temp files.
• Consolidated to a single .reuse.toml at the repo root for license compliance.
• Set up CI workflows for:
  • Backend → golangci-lint
  • Frontend → lint, prettier, build checks

---