Add browsers for tests: Playwright, Firefox, geckodriver, headless Chrome (closes #10)

Firefox comes from Mozilla's own apt repo since Ubuntu's is a snap that
does not run in a container. geckodriver is matched to it at build time.
Playwright installs its Chromium and Firefox, skipping WebKit which has
no linux-arm64 build; Playwright's Chromium is exposed as the headless
Chrome.

Model: deepseek-v4-pro

bin/test

@@ -35,6 +35,10 @@ vips -l | grep -i heif > /dev/null
     valkey-server --version
     valkey-cli --version
     psql --version
+    playwright --version
+    firefox --version
+    geckodriver --version
+    chromium --version
 '
 
 echo "== $img: runit service lifecycle"