feat: add Playwright E2E testing setup with POM and testids

- Install @playwright/test and Chromium browser
- Create playwright.config.ts with dev server integration
- Add data-testid attributes to LoginPage, DashboardPage, ServerCard, Sidebar
- Exclude tests-e2e from vitest config
- Create Page Object Models: LoginPage, DashboardPage
- Add 18 E2E tests: 6 login flow, 12 dashboard (happy, empty, error states)
- Add test:e2e and test:e2e:ui scripts to package.json

frontend/tests-e2e/pages/LoginPage.ts

@@ -0,0 +1,40 @@
+import { Page, Locator } from "@playwright/test";
+
+export class LoginPage {
+  readonly page: Page;
+  readonly card: Locator;
+  readonly form: Locator;
+  readonly usernameInput: Locator;
+  readonly passwordInput: Locator;
+  readonly submitButton: Locator;
+  readonly errorMessage: Locator;
+
+  constructor(page: Page) {
+    this.page = page;
+    this.card = page.locator('[data-testid="login-card"]');
+    this.form = page.locator('[data-testid="login-form"]');
+    this.usernameInput = page.locator('[data-testid="login-username"]');
+    this.passwordInput = page.locator('[data-testid="login-password"]');
+    this.submitButton = page.locator('[data-testid="login-submit"]');
+    this.errorMessage = page.locator('[data-testid="login-error"]');
+  }
+
+  async goto() {
+    await this.page.goto("/login");
+    await this.card.waitFor({ state: "visible" });
+  }
+
+  async login(username: string, password: string) {
+    await this.usernameInput.fill(username);
+    await this.passwordInput.fill(password);
+    await this.submitButton.click();
+  }
+
+  async getErrorMessage() {
+    return this.errorMessage.textContent();
+  }
+
+  async hasErrorMessage() {
+    return this.errorMessage.isVisible();
+  }
+}