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/playwright.config.ts

@@ -0,0 +1,60 @@
+import { defineConfig, devices } from "@playwright/test";
+
+/**
+ * CookieBridge Admin Frontend - Playwright E2E Test Configuration
+ *
+ * Prerequisites: RCA-12 (scaffold), RCA-13 (API), RCA-14 (login),
+ * RCA-15 (dashboard), RCA-16 (cookies), RCA-17 (devices), RCA-18 (settings)
+ * must all be complete before running these tests.
+ *
+ * Usage:
+ *   npm run test:e2e          — run all tests headless
+ *   npm run test:e2e:ui       — interactive UI mode
+ *   npm run test:e2e:headed   — run with browser visible
+ */
+export default defineConfig({
+  testDir: "./tests/e2e",
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: process.env.CI ? 1 : undefined,
+  reporter: [["html", { open: "never" }], ["list"]],
+  use: {
+    baseURL: process.env.BASE_URL ?? "http://localhost:5173",
+    trace: "on-first-retry",
+    screenshot: "only-on-failure",
+    video: "retain-on-failure",
+  },
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+    {
+      name: "firefox",
+      use: { ...devices["Desktop Firefox"] },
+    },
+    {
+      name: "webkit",
+      use: { ...devices["Desktop Safari"] },
+    },
+    {
+      name: "mobile-chrome",
+      use: { ...devices["Pixel 5"] },
+    },
+    {
+      name: "mobile-safari",
+      use: { ...devices["iPhone 12"] },
+    },
+    {
+      name: "tablet",
+      use: { ...devices["iPad Pro 11"] },
+    },
+  ],
+  webServer: {
+    command: "npm run dev",
+    url: "http://localhost:5173",
+    reuseExistingServer: !process.env.CI,
+    timeout: 30_000,
+  },
+});