fix(tests): scope admin settings heading locator to the guests section

[ernolf]

The test (Playwright) job has been red on main and on every PR since around 2026-06-11. It is not caused by any guests change: between the last green run (2026-06-09) and the first red one (2026-06-11) the only commits were a Transifex l10n update and an AUTHORS entry, and admin-settings.spec.ts itself is unchanged since it was added.

The assertion trips Playwright strict mode:

getByRole('heading', { name: 'Guests', exact: true }) resolved to 2 elements:
  1) <h1 class="hidden-visually" id="page-heading-level-1">Guests</h1>
  2) <h2 class="settings-section__name">Guests</h2>  (inside #guest-settings)

The settings framework renders a visually-hidden page heading <h1> carrying the active section name ("Guests"), which collides with our own section <h2>. That <h1> is server-side and predates this app; nothing in this repo changed. The test job boots the continuous-integration-shallow-server image, a shallow clone of nextcloud/server master taken at image build time in nextcloud/docker-ci; it was last rebuilt on 2026-06-10 (build) and the first guests run afterwards is the first red one: green 2026-06-09 → red 2026-06-11, red on every run since.

This scopes the locator to the app's #guest-settings container, which is exactly what Playwright's own error output suggests, so it matches only our section heading. No change to app behaviour.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI
  Assisted-by: ClaudeCode:claude-opus-4-8

[ernolf]

The test (Playwright) job is green again ✅

Scoping the assertion to the app's own #guest-settings container makes it resolve to exactly one heading (our section <h2>Guests</h2>), so the strict-mode collision with the framework's hidden page <h1> is gone. All other checks pass as well.

Ready for review. 👀