test(web): add Playwright E2E and admin API test suite for RCA-19

Prepares the full QA test infrastructure for the admin frontend before
all prerequisite feature tasks (RCA-12–18) are complete.

- playwright.config.ts: 6 browser/device projects (Chromium, Firefox,
  WebKit, mobile Chrome, mobile Safari, tablet)
- tests/e2e/01-login.spec.ts: login form, route guards, setup wizard
- tests/e2e/02-dashboard.spec.ts: stats cards, device list, quick actions
- tests/e2e/03-cookies.spec.ts: cookie list, search, detail panel, delete
- tests/e2e/04-devices.spec.ts: device cards, revoke flow, status filter
- tests/e2e/05-settings.spec.ts: three-tab layout, save/error toasts
- tests/e2e/06-responsive.spec.ts: no horizontal scroll on mobile/tablet
- tests/api/admin-api.spec.ts: REST API contract tests for all /admin/* endpoints
- helpers/auth.ts: loginViaUI + loginViaAPI helpers
- helpers/mock-api.ts: route intercept fixtures for all pages

Co-Authored-By: Paperclip <noreply@paperclip.ing>

web/package.json

@@ -6,7 +6,12 @@
   "scripts": {
     "dev": "vite",
     "build": "vue-tsc --noEmit && vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test:e2e": "playwright test tests/e2e/",
+    "test:e2e:ui": "playwright test tests/e2e/ --ui",
+    "test:e2e:headed": "playwright test tests/e2e/ --headed",
+    "test:api": "playwright test tests/api/ --project=chromium",
+    "test:all": "playwright test"
   },
   "dependencies": {
     "@headlessui/vue": "^1.7.0",