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/tests/e2e/helpers/auth.ts

@@ -0,0 +1,52 @@
+import { type Page, type APIRequestContext, expect } from "@playwright/test";
+
+export const TEST_ADMIN = {
+  username: process.env.TEST_ADMIN_USER ?? "admin",
+  password: process.env.TEST_ADMIN_PASS ?? "testpassword123",
+};
+
+/**
+ * Log in via the UI login form and wait for the dashboard to load.
+ */
+export async function loginViaUI(page: Page): Promise<void> {
+  await page.goto("/login");
+  await page.getByLabel(/username/i).fill(TEST_ADMIN.username);
+  await page.getByLabel(/password/i).fill(TEST_ADMIN.password);
+  await page.getByRole("button", { name: /log in|sign in/i }).click();
+  await expect(page).toHaveURL(/\/dashboard/);
+}
+
+/**
+ * Log in via the admin API directly and store the token in localStorage.
+ * Faster than UI login for tests that only need an authenticated session.
+ */
+export async function loginViaAPI(
+  page: Page,
+  request: APIRequestContext,
+): Promise<string> {
+  const resp = await request.post("/admin/auth/login", {
+    data: { username: TEST_ADMIN.username, password: TEST_ADMIN.password },
+  });
+  expect(resp.status()).toBe(200);
+  const body = await resp.json();
+  expect(body).toHaveProperty("token");
+
+  await page.goto("/");
+  await page.evaluate(
+    ({ token }) => localStorage.setItem("admin_token", token),
+    { token: body.token as string },
+  );
+  return body.token as string;
+}
+
+/**
+ * Log out via the UI and confirm redirect to /login.
+ */
+export async function logoutViaUI(page: Page): Promise<void> {
+  // Common patterns: a "Logout" button in the nav/header
+  const logoutBtn = page
+    .getByRole("button", { name: /log ?out|sign ?out/i })
+    .or(page.getByRole("link", { name: /log ?out|sign ?out/i }));
+  await logoutBtn.click();
+  await expect(page).toHaveURL(/\/login/);
+}