Add backend unit/integration tests, Playwright e2e tests, README, cookie Secure fix

frontend/playwright.config.ts

@@ -0,0 +1,19 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './tests/e2e',
+  fullyParallel: true,
+  retries: process.env.CI ? 1 : 0,
+  reporter: [['list']],
+  use: {
+    baseURL: 'http://localhost:5173',
+    trace: 'on-first-retry'
+  },
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:5173',
+    reuseExistingServer: !process.env.CI,
+    timeout: 30000
+  },
+  projects: [{ name: 'chromium', use: { ...devices['Desktop Chrome'] } }]
+});