Add a webkit project to the Playwright config so the hermetic mock-mode specs run
in both Chromium and WebKit, and install both browsers in CI. WebKit's Debian build
runs headless without extra host system libraries (verified locally: smoke + zoom
pass in webkit); the workflow comment records the one-time host install-deps fallback
if a runner ever lacks a library. Desktop WebKit does not reproduce iOS Safari's text
auto-inflation, so the app.css text-size-adjust guard stays outside e2e coverage.