Stage 7: run the e2e suite in WebKit too (Safari-engine coverage)

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.

ui/playwright.config.ts

@@ -18,5 +18,12 @@ export default defineConfig({
     reuseExistingServer: !process.env.CI,
     timeout: 60_000,
   },
-  projects: [{ name: 'chromium', use: { ...devices['Desktop Chrome'] } }],
+  // Run the same hermetic specs in Chromium and WebKit (Safari's engine) so the UI is
+  // exercised in both rendering/JS engines. Note: desktop WebKit on Linux does not
+  // reproduce iOS Safari's text auto-inflation, so the `text-size-adjust` guard in
+  // app.css is not regression-covered here — but engine-level CSS/JS differences are.
+  projects: [
+    { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
+    { name: 'webkit', use: { ...devices['Desktop Safari'] } },
+  ],
 });