Add a headless browser to the default template

[defkode]

Problem

The default template is a Rails sandbox, but there's no browser in it. That
means an agent working in the sandbox cannot run system tests and cannot
verify that a page renders — it can only reason about the code it just wrote.
For a Rails template this is the single largest capability gap: the agent's
feedback loop stops at the model and controller layer.

Proposal

npx playwright install --with-deps chromium

This gets two things at once: a Capybara-drivable Chrome for system tests, and
a scriptable browser the agent can drive directly to check a rendered page.

Do not use apt install chromium on Ubuntu — it's a snap transitional
package and is painful in a container.

Considerations

• --with-deps pulls a meaningful number of shared libraries; this is the
  heaviest single addition proposed for the template. If image size matters,
  this may deserve its own template (rails-e2e?) rather than living in
  default.
• Install at provision time or lazily on first use? Provisioning happens once
  per sandbox, so doing it upfront keeps the first test run fast — at the cost
  of a slower orbx up.
• Chromium only, or also Firefox/WebKit? Chromium alone covers the common
  Capybara setup.
• Worth checking whether selenium-webdriver / cuprite is what the target
  Rails version's generated application_system_test_case.rb actually expects,
  and matching that.

Acceptance

A generated Rails app's default system test passes inside a fresh sandbox with
no additional setup.