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>

e2e/test_comments.py

@@ -54,6 +54,6 @@ def test_comments_section_absent_when_disabled(page: Page, base_url: str) -> Non
     )
     # Confirm we're on the right page
     expect(page.get_by_role("heading", level=1)).to_have_text("No Comments Article")
-    # Comments section must be absent
-    expect(page.get_by_role("heading", name="Comments")).to_have_count(0)
+    # Comments section must be absent — exact=True prevents matching "No Comments Article" h1
+    expect(page.get_by_role("heading", name="Comments", exact=True)).to_have_count(0)
     expect(page.get_by_role("button", name="Post comment")).to_have_count(0)