Add wp-admin smoke test

[conatus]

Summary

• Adds a Playwright spec that proves the plugin's single wp-admin surface (the Carbon Fields "Join" settings page) is not broken.
• Motivation: the most recent release fixed a bug where clicking around the backend with the plugin active produced errors. No automated test existed to catch that class of regression.
• Out of scope (future tier): the Gutenberg block editor surface.

What it covers

1. Tab navigation — iterates all 6 settings-page tabs (Features, Membership Plans, Theme, Copy, Integrations, Logging), asserts each loads without a PHP error or 5xx, and that the tab panel actually changes.
2. Carbon Fields round-trip — updates organisation_name on the Copy tab, clicks Save, confirms settings-updated=true redirect, reloads, and asserts the database value persisted. Restores the original value so local re-runs are idempotent.

Notable implementation decisions

• scripts/setup.php now pre-populates the three required Copy-tab fields plus one membership plan so Carbon Fields' save button and cross-validator stay unlocked in the test environment.
• A test.afterAll hook restores stripe_direct_debit_only=true via wp eval after the round-trip, because Carbon Fields saves the entire container and would otherwise wipe options that aren't mirrored as visible form fields.
• The tab test filters the pre-existing "Please call Stripe() with your publishable key" SDK warning — that fires any time the admin loads with empty credentials and is not a regression signal.

Test plan

• cd packages/join-e2e && npx wp-env start
• bash scripts/seed.sh
• npx playwright test tests/wp-admin-smoke.spec.ts --reporter=list — both tests pass
• npx playwright test --reporter=list — full suite passes (59 tests across 9 spec files)
• Break the admin deliberately (e.g. add trigger_error(..., E_USER_ERROR) to Settings.php::registerSettings) and re-run — both tests must fail. Revert and confirm green again.

Suite count

Was: 57 tests / 8 spec files
Now: 59 tests / 9 spec files