test: E2E smoke tests + Playwright browser tests (23 tests)

smoke.sh:
- Full E2E smoke test: health, npm proxy/publish/security, Maven, PyPI, Docker, Raw, UI, mirror CLI
- Self-contained: starts NORA, runs tests, cleans up

Playwright (tests/e2e/):
- Dashboard: page load, registry sections visible, npm count > 0, Docker stats
- npm: URL rewriting, scoped packages, tarball download, publish, immutability, security
- Docker: v2 check, catalog, manifest push/pull, tags list
- Maven: proxy download, upload
- PyPI: simple index, package page
- Raw: upload and download
- Health, metrics, OpenAPI endpoints

All 23 tests pass in 4.7s against live NORA instance.

tests/e2e/playwright.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './tests',
+  timeout: 30000,
+  retries: 1,
+  use: {
+    baseURL: process.env.NORA_URL || 'http://localhost:4000',
+    screenshot: 'only-on-failure',
+    trace: 'retain-on-failure',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: { browserName: 'chromium' },
+    },
+  ],
+});