Add CI workflow for backend, frontend, and Docker builds

[Davictory2003]

No description provided.

[Copilot]

Pull request overview

This PR adds a GitHub Actions CI workflow to automate testing and building for the full-stack cheat-sheet application. The workflow is configured to run on pushes and pull requests to the main branch, providing automated quality checks for both backend and frontend components as well as Docker image builds.

Changes:

• Added CI workflow with three jobs: backend testing (Python/Django), frontend building (Node.js/React), and Docker image building
• Configured Python 3.14 with pip caching for backend dependencies and testing
• Configured Node.js 20 with npm caching for frontend dependencies and building

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The frontend has ESLint installed (see frontend/package.json:19-21) but there is no linting step in the CI workflow and no "lint" script in package.json. Consider adding a "lint" script to frontend/package.json and adding a linting step before the build step in this workflow to catch code quality issues early.

Suggested change

	- name: Lint
	run: npm run lint

[Davictory2003]

Saving this for another commit