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

build.sh

@@ -77,7 +77,7 @@ nvm install --lts
 nvm alias default "lts/*"
 nvm use default
 set -ux
-npm install -g npm@latest corepack wrangler prettier
+npm install -g npm@latest corepack wrangler prettier playwright
 corepack enable
 # Pre-populate the corepack cache so projects that pin a package manager
 # do not download at run time. The last yarn installed becomes the default.
@@ -90,6 +90,42 @@ pnpm --version
 yarn --version
 nvm cache clear
 
+# --- browsers for tests: Firefox, geckodriver, Playwright --------------------
+# Ubuntu's firefox is a snap that does not run in a container, so use
+# Mozilla's own apt repository for the real binary.
+install -d -m 0755 /etc/apt/keyrings
+curl -fsSL https://packages.mozilla.org/apt/repo-signing-key.gpg \
+    -o /etc/apt/keyrings/packages.mozilla.org.asc
+echo "deb [signed-by=/etc/apt/keyrings/packages.mozilla.org.asc] https://packages.mozilla.org/apt mozilla main" \
+    > /etc/apt/sources.list.d/mozilla.list
+cat > /etc/apt/preferences.d/mozilla <<'EOF'
+Package: *
+Pin: origin packages.mozilla.org
+Pin-Priority: 1000
+EOF
+apt-get update
+apt-get -y install --no-install-recommends -t mozilla firefox
+
+# geckodriver, at its latest release (independent of the Firefox version).
+gd="$(latest_tag mozilla/geckodriver)"
+case "$arch" in
+    arm64) gd_arch=linux-aarch64 ;;
+    *) gd_arch=linux64 ;;
+esac
+curl -fsSL "https://github.com/mozilla/geckodriver/releases/download/${gd}/geckodriver-${gd}-${gd_arch}.tar.gz" |
+    tar -C /usr/local/bin -xz geckodriver
+
+# Playwright's Chromium and Firefox. WebKit is skipped because no repository
+# uses it today. Playwright's Chromium is also the headless Chrome for
+# chromedp-style clients, exposed as `chromium` on PATH.
+playwright install --with-deps chromium firefox
+chromiums=( "$PLAYWRIGHT_BROWSERS_PATH"/chromium-*/chrome-linux*/chrome )
+if [ "${#chromiums[@]}" -ne 1 ] || [ ! -x "${chromiums[0]}" ]; then
+    echo "error: expected exactly one chromium binary, found ${#chromiums[@]}" >&2
+    exit 1
+fi
+ln -s "${chromiums[0]}" /usr/local/bin/chromium
+
 # --- pyenv, Python (latest), pip, virtualenv ---------------------------------
 git clone -q --depth 1 --branch "$PYENV_GIT_TAG" https://github.com/pyenv/pyenv.git "$PYENV_ROOT"
 rm -rf "$PYENV_ROOT/.git"
@@ -141,6 +177,10 @@ COREPACK_ENABLE_NETWORK=0 yarn --version
 COREPACK_ENABLE_NETWORK=0 pnpm --version
 wrangler --version
 prettier --version
+playwright --version
+firefox --version
+geckodriver --version
+chromium --version
 python3 --version
 pip --version
 virtualenv --version