CI: install the browser the prerender pass needs

pnpm build ends in prerender, which drives a real chromium. Without the install
step the build failed on its very last action, after typecheck, the contract
gates and the tests had all gone green — the most expensive place to discover a
missing dependency.

.github/workflows/ci.yml

@@ -26,6 +26,12 @@ jobs:
           test "$(pnpm -v)" = "11.21.0" || { echo "pnpm $(pnpm -v) != 11.21.0"; exit 1; }
 
       - run: CI=true pnpm install --frozen-lockfile
+
+      # `pnpm build` ends in the prerender pass, which drives a real browser.
+      # Without this the build fails at the very last step, after everything
+      # else has passed, with a Playwright message about installing browsers.
+      - run: pnpm exec playwright install --with-deps chromium
+
       - run: pnpm typecheck
       - run: pnpm check
       - run: pnpm test