Stabilize PWA offline system spec and offline banner behavior

[Copilot]

The failing System specs / Test job came from the PWA offline flow being timing-sensitive: the cached homepage could load offline before the service worker reliably controlled the page, and the offline banner visibility depended on a one-time page-load check.

• Harden service-worker timing in the PWA spec

  • Wait for navigator.serviceWorker.ready
  • Revisit the homepage so the service worker can take control
  • Wait for controllerchange before switching the browser offline

• Make the offline banner reactive instead of load-only

  • Override the Decidim offline banner partial
  • Recompute banner visibility on offline, online, and pageshow
  • Keep the banner visible when serving the cached page offline, not only on initial DOMContentLoaded

• Resulting behavior

  • The PWA spec no longer assumes the service worker is immediately active after the first visit
  • The UI now reflects offline state transitions after navigation, which is the condition exercised by the failing system spec

visit decidim.root_path
wait_for_service_worker_to_be_ready

visit decidim.root_path
wait_for_service_worker_to_control_page

with_browser_in_offline_mode do
  visit decidim.root_path
  expect(page).to have_content("Your network is offline...")
end