Add GitHub Actions validation workflow

[Copilot]

Adds pull request and main CI coverage for backend and frontend validation.

• Python

  • Django test suite
  • Ruff lint and formatting checks

• Frontend

  • Runs npm run fix before validation so auto-fixable issues do not block CI
  • Vitest, ESLint, Prettier, and production build checks

- name: Apply automatic fixes
  run: npm run fix
- name: Run tests
  run: npm run test