feat: add Playwright configuration and initial e2e test for authentication

- Created Playwright configuration file to set up testing environment.
- Added a new e2e test for user authentication in login.spec.ts.
- Updated tsconfig.node.json to include playwright.config.ts.
- Enhanced vite.config.ts to include API proxying for backend integration.
- Added a placeholder for last run test results in .last-run.json.

frontend/tests/e2e/login.spec.ts

@@ -0,0 +1,26 @@
+import { expect, test, type Page } from '@playwright/test';
+
+const demoCredentials = {
+  username: process.env.DEMO_USERNAME ?? 'demo',
+  password: process.env.DEMO_PASSWORD ?? 'railgame123',
+};
+
+test.describe('Authentication', () => {
+  test('allows the demo user to sign in and view the network snapshot', async ({
+    page,
+  }: {
+    page: Page;
+  }) => {
+    await page.goto('/');
+
+    await expect(page.getByRole('heading', { name: 'Sign in' })).toBeVisible();
+
+    await page.getByLabel('Username').fill(demoCredentials.username);
+    await page.getByLabel('Password').fill(demoCredentials.password);
+    await page.getByRole('button', { name: 'Sign in' }).click();
+
+    await expect(page.getByRole('heading', { name: 'Network Snapshot' })).toBeVisible();
+    await expect(page.getByRole('heading', { name: 'Stations' })).toBeVisible();
+    await expect(page.getByRole('button', { name: 'Sign out' })).toBeVisible();
+  });
+});