* fix: stabilize and diagnose browser CI gate
- Replace networkidle screenshot wait (5s client timeout, flaky with
long-lived connections) with readyState + fonts-loaded wait and fixed
settle in a StableScreenshot helper.
- Export standalone diff/expected/actual PNGs on visual mismatch so CI
artifacts are directly viewable (vendor only writes an HTML diff view).
- Create .env in CI test jobs; without it Laravel's env bootstrap emits a
file_get_contents warning on every test.
- Bump checkout/cache actions to v5 (Node 20 deprecation).
- Gitignore tests/Browser/Screenshots.
* docs: track browser CI gate task status in tasks.md
