Add Playwright test suite (51 tests, all passing)

- tests/api.spec.js: 17 tests covering public booking API and admin CRUD
  (health, POST bookings, auth rejection, stats, filters, PATCH, CSV export)
- tests/booking.spec.js: 12 tests covering the 7-step booking wizard
  (open/close, date validation, step progression, contract scroll+signature,
  payment checkbox, full end-to-end booking flow)
- tests/dashboard.spec.js: 15 tests covering the admin dashboard
  (login, wrong passphrase, Enter key, sign out, overview stats, calendar
  month nav, bookings table, search filter, row expand, CSV download)
- tests/portfolio.spec.js: 7 tests covering the portfolio site
  (nav, book button, portfolio section, config, health, theme color)
- playwright.config.js: Chromium headless, BASE_URL + ADMIN_SECRET via env
- Fix dangling client-email input outside step-panel (was visible, making
  the booking modal taller and hiding the Next button in headless mode)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -34,3 +34,6 @@ docker-compose.override.yml
 api/data/*.db
 api/signed-contracts/*.pdf
 api/.env
+test-results/
+playwright-report/
+node_modules/