Add Vitest unit tests and Playwright e2e suite for frontend

Replaces the ad-hoc test-e2e.mjs scripts with a proper test stack:
Vitest covers $lib (api, auth, signalr) with mocked fetch/SignalR/navigation,
and @playwright/test covers auth, stores, lists, recipes, and SignalR realtime
sync between two browser contexts. Tests use uniqueName() for every entity
since the backend has no per-test reset.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/frontend/playwright.config.ts

@@ -0,0 +1,26 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+	testDir: './tests/e2e',
+	fullyParallel: false,
+	forbidOnly: !!process.env.CI,
+	retries: 0,
+	reporter: 'html',
+	timeout: 30_000,
+	expect: {
+		timeout: 8_000
+	},
+	use: {
+		baseURL: 'http://localhost:5173',
+		trace: 'on-first-retry'
+	},
+	projects: [
+		{
+			name: 'chromium',
+			use: {
+				...devices['Pixel 5'],
+				viewport: { width: 390, height: 844 }
+			}
+		}
+	]
+});