E2E testing for the p5.js web editor

The p5.js Web Editor has no end-to-end tests. As the team works through a TypeScript migration and backend refactor, regressions in core user flows can go undetected and only surface through manual review. This project builds a complete E2E testing suite using Playwright, integrated into GitHub Actions so every PR into the develop branch is automatically validated against real user behaviour. The three deliverables are a working CI pipeline that runs the E2E suite on every PR, a set of prioritised user flows covering unauthenticated, authenticated, and state-transition scenarios, and contributor documentation so the suite can be extended after GSoC without the original author present.