P3: Add GitHub Actions CI for Flutter + PHP

[Kinin-Code-Offical]

The project lacks an automated CI pipeline to ensure code quality and catch regressions. The existing CI files should be audited and fixed.

Acceptance Criteria:

• A GitHub Actions workflow file in .github/workflows/ must pass successfully.
• The workflow must trigger on push and pull_request.
• It must have separate jobs for rontend-ci and �ackend-ci.
• The backend job must run composer install and phpunit.
• The frontend job must run lutter analyze and lutter test.