fix(frontend): prevent using mock feature flags (#10792)

## Changes 🏗️

Make sure `NEXT_PUBLIC_PW_TEST` is set only when running Playwright.
This forces the app to use "mock" feature flags, so the tests run stable
and predictable despite changes on LaunchDarkly.

## Checklist 📋

### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x]  should not have `PW_TEST=true` ...

### For configuration changes:

None