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.

.gitignore

@@ -18,6 +18,7 @@
 /public/fonts-manifest.dev.json
 /public/hot
 /public/storage
+/tests/Browser/Screenshots
 /storage/*.key
 /storage/pail
 /vendor