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

build.sh

@@ -68,7 +68,7 @@ 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.
@@ -81,6 +81,33 @@ 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
+apt-get update
+apt-get -y install --no-install-recommends -t mozilla firefox
+
+# geckodriver, matched to Firefox at its current release.
+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}-${gd_arch}.tar.gz" |
+    tar -C /usr/local/bin -xz geckodriver
+
+# Playwright's Chromium and Firefox. WebKit has no linux-arm64 build, so it
+# is skipped. Playwright's Chromium is also the headless Chrome for
+# chromedp-style clients, exposed as `chromium` on PATH.
+playwright install --with-deps chromium firefox
+ln -s "$(ls -d "$PLAYWRIGHT_BROWSERS_PATH"/chromium-*/chrome-linux*/chrome | head -n1)" \
+    /usr/local/bin/chromium
+
 # --- pyenv, Python (latest), pip, virtualenv ---------------------------------
 git clone -q --depth 1 --branch "$PYENV_GIT_TAG" https://github.com/pyenv/pyenv.git "$PYENV_ROOT"
 rm -rf "$PYENV_ROOT/.git"
@@ -129,6 +156,10 @@ 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