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>

playwright.config.js

@@ -0,0 +1,26 @@
+// @ts-check
+const { defineConfig } = require('@playwright/test');
+
+const BASE_URL = process.env.BASE_URL || 'http://192.168.1.192:8080';
+const ADMIN_SECRET = process.env.ADMIN_SECRET || '9yPuNfYjy5kisRKrowhmH42PTeEgH';
+
+module.exports = defineConfig({
+  testDir: './tests',
+  timeout: 30_000,
+  expect: { timeout: 8_000 },
+  retries: 1,
+  workers: 1,
+  reporter: [['list'], ['html', { outputFolder: 'playwright-report', open: 'never' }]],
+  use: {
+    baseURL: BASE_URL,
+    headless: true,
+    screenshot: 'only-on-failure',
+    video: 'retain-on-failure',
+  },
+  projects: [
+    { name: 'chromium', use: { browserName: 'chromium' } },
+  ],
+  // Expose config values to tests via env
+  globalSetup: undefined,
+  env: { ADMIN_SECRET, BASE_URL },
+});