Add Playwright E2E test infrastructure to argocd-plugin

- Add @playwright/test and playwright as devDependencies
- Add e2e and e2e:headed scripts
- Add playwright.config.ts
- Add basic e2e test and auth.setup.ts

This fixes the E2E workflow which was failing at the Playwright
install step because the project lacked Playwright dependencies.

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

e2e/basic.spec.ts

@@ -0,0 +1,16 @@
+import { test, expect } from '@playwright/test';
+
+test.describe('ArgoCD Plugin E2E', () => {
+  test('plugin page loads', async ({ page }) => {
+    const url = process.env.HEADLAMP_URL;
+    if (!url) {
+      throw new Error('HEADLAMP_URL must be set');
+    }
+
+    await page.goto(url);
+    await page.waitForLoadState('networkidle');
+
+    const title = await page.title();
+    expect(title).toBeTruthy();
+  });
+});