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

Firefox comes from Mozilla's own apt repo, pinned above Ubuntu's snap-only
package. geckodriver is at its latest release, independent of the Firefox
version. Playwright installs its Chromium and Firefox; WebKit is skipped
because no repository uses it today. Playwright's Chromium is exposed as
the headless Chrome.

Model: deepseek-v4-pro

bin/test

@@ -39,6 +39,10 @@ docker run --rm "$img" bash -ec '
     valkey-cli shutdown nosave
     psql --version
     postgres --version
+    playwright --version
+    firefox --version
+    geckodriver --version
+    chromium --version
 '
 
 echo "== $img: runit service lifecycle"