test(ui): Playwright RLS sweep — real-browser proof, both personas

Phase 5.5 (plan §7.5) first increment: the SSR/HTMX frontend is swept
in a real browser (official Playwright image, no host browsers) for
BOTH personas — testuser (admin) and rls-phone (restricted; Audiobooks
hidden from them via the visibility API).

Green checks: auth rejection + session establishment; admin dashboard
renders sections incl. the hidden-from-others library; restricted
dashboard renders real sections (Recently Added present — not a
vacuous landing-page pass) with ZERO hidden-library traces; search and
media payloads leak nothing for the restricted persona.

Harness lessons encoded: / is a landing page (dashboard lives at
/dashboard); the login endpoint rate-limits rapid attempts so sessions
are established once per persona and reused via storageState — a sweep
must not look like a brute-force run; getByText visibility assertions
are wrong for <option> text (library switcher) — body-textContent
assertions instead.

COVERAGE.md tracks the full 14-area sweep per the plan; the remaining
areas (reader, collections CRUD, admin matrix, upload roundtrip) are
enumerated and pending their sweep runs.

tests/ui/.gitignore

@@ -0,0 +1,4 @@
+node_modules/
+package-lock.json
+test-results/
+playwright-report/