Hosted player: scheduled check that trace.webdriver.io opens a trace #511

Part of #494.

Problem

Once trace.webdriver.io is live, nothing tells us when it is down or when a deploy breaks opening a trace. Vercel reports build failures, but a deploy that builds and then fails in the browser goes unnoticed until a user reports it.

Proposal

A scheduled GitHub Actions workflow that loads the production page and opens a committed fixture zip through it, checking that the player renders the trace's actions, and opens an issue or notifies maintainers when it fails. It runs on a schedule and after each production deploy.

The page itself keeps no visitor analytics, by design: a trace never leaves the browser, and the check must not change that.