fix: run E2E tests properly with mounted browsers and real postgres

- Mount /opt/playwright-tools/browsers into web container (docker-compose.yml
  and CI docker run) — never download browsers, use the ones on this host
- Set PLAYWRIGHT_BROWSERS_PATH in all container envs (compose + CI)
- Drop 'playwright install chromium' steps from pr-e2e and nightly-e2e jobs
- Bump playwright requirement to ~1.57.0 to match the installed browser builds
- Fix seed_e2e_content: de-duplicate default Site entries left by unit test
  fixtures so Wagtail always routes to the seeded home page
- Fix test_comments_section_absent_when_disabled: use exact=True on heading
  locator to avoid matching 'No Comments Article' h1 as 'Comments' heading
- Fix test_copy_link_button_updates_text: use [data-copy-link] data-attr
  locator (stable across text change) and force-override clipboard.writeText
  via page.evaluate() rather than relying on init_script polyfill

E2E suite verified locally: 34 passed via docker compose exec

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/core/management/commands/seed_e2e_content.py

@@ -129,5 +129,7 @@ class Command(BaseCommand):
         site.is_default_site = True
         site.site_name = "No Hype AI"
         site.save()
+        # Remove any other conflicting default-site entries left by test fixtures
+        Site.objects.exclude(pk=site.pk).filter(is_default_site=True).update(is_default_site=False)
 
         self.stdout.write(self.style.SUCCESS("Seeded E2E content."))