Adds a Playwright spec gated on VISUAL=1 with one snapshot per
(mobile/tablet/desktop × light/dark) = 6 baselines. Snapshots stay
skipped in CI until the baseline set is captured and committed —
running `playwright test --update-snapshots briefwechsel-rows`
against a seeded backend generates them.
Structural check runs unconditionally so the file is wired into CI
today rather than waiting for the baseline capture step.
Refs #305
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>