Add a Firefox browser smoke journey

[argentquest]

Goal

Extend critical browser confidence beyond Chromium with a focused Firefox smoke journey while keeping GitHub Actions usage modest.

Acceptance criteria

• reuse the existing Playwright setup and web server
• cover first visit, public theme selection, starting a level, and opening About
• run locally through a documented command
• choose a CI schedule or trigger that avoids multiplying every pull-request run unnecessarily
• document expected runtime and any Firefox-specific exceptions
• keep Chromium as the required pull-request journey unless maintainers explicitly change branch protection

[kimberly-WY]

One thing worth trying: setting the timeout at the transport layer rather than the request level — avoids the race condition.