fix: resolve ruff/mypy lint errors and fix E2E test failures

- Use datetime.timedelta instead of timezone.timedelta (mypy)
- Fix import ordering (ruff I001)
- Fix admin sidebar E2E selector: use #wagtail-sidebar (Wagtail 7)
- Set deterministic published_date on seeded E2E articles for stable ordering
- Fix nightly test strict-mode violation: exact=True for Comments heading

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

e2e/test_admin_experience.py

@@ -19,7 +19,7 @@ def admin_login(page: Page, base_url: str) -> None:
 def test_articles_menu_item_visible(page: Page, base_url: str) -> None:
     """The admin sidebar should contain an 'Articles' menu item."""
     admin_login(page, base_url)
-    sidebar = page.locator("[data-side-panel]").first
+    sidebar = page.locator("#wagtail-sidebar")
     articles_link = sidebar.get_by_role("link", name="Articles")
     expect(articles_link).to_be_visible()
 
@@ -47,8 +47,8 @@ def test_article_editor_has_tabs(page: Page, base_url: str) -> None:
     """The article editor should have Content, Metadata, Publishing, and SEO tabs."""
     admin_login(page, base_url)
     page.goto(f"{base_url}/cms/articles/", wait_until="networkidle")
-    # Click the first article to edit it
-    page.get_by_text("Nightly Playwright Journey").first.click()
+    # Click the first article title link to edit it
+    page.get_by_role("link", name="Nightly Playwright Journey").first.click()
     page.wait_for_load_state("networkidle")
     expect(page.get_by_role("tab", name="Content")).to_be_visible()
     expect(page.get_by_role("tab", name="Metadata")).to_be_visible()